General election polling can be inconsistent in terms of numbers, however, Labour is still performing poorly overall and is in particular losing its grip on older and more rural voters even in places traditionally inclined towards Labour. With the mines and old industry gone, demographic change is happening fast and many former mining towns are becoming commuter territory to major cities.
Before this snap general election was announced, Labour's loss of Copeland in a by-election to the Conservatives gave hints that other long-held Labour seats could fall (Copeland fka Whitehaven had been held by Labour since 1935 before said by-election): https://greensocialistalan.blogspot.co.uk/2017/02/my-analysis-of-local-by-election.html
With the gap between the Conservatives and Labour wider than ever, and with UKIP voters likely to turn towards the Conservatives especially in those types of constituencies (common factors include high proportion of people of socio-economic classes C2, D, and E; lower than average annual salaries, unemployment above average, once host to an industry that has since disappeared from the area, and overall lower levels of educational attainment).
With this in mind, here are my predictions for 15 marginal/semi-marginal seats which Labour have held continuously (excluding by-elections and defections e.g. Lab-SDP and accounting for closest predecessor seats) since at least 1945 (and often earlier than that), and which have similar characteristics with each other making them unfavourable for Labour at present:
Newcastle-under-Lyme. Its Labour MP, Paul Farrelly, was one of only 13 to vote against dissolving Parliament early, and for good reason. Even though Labour has held this seat continuously since 1919, it is looking more vulnerable than ever due to the demographic changes that have taken place and dissatisfaction with Labour in Staffordshire as a whole. The Conservatives have been consolidating their support in Newcastle-under-Lyme and it has been shifting away from Labour for some time as the potteries have declined. Dead cert Conservative gain.
North East Derbyshire. Held by Labour since 1935, it is fast becoming commuter territory for people working in Sheffield as the ex-miner demographic fades away, particularly in the town of Dronfield. The current Labour majority in this seat is as low as in 1983 and with local elections being strictly between the Conservatives and Labour in practice, and the area becoming more prosperous, a Conservative victory will be rather easy. Dead cert Conservative gain.
Walsall North. Walsall North has, apart from a notable by-election in 1976, continuously evaded the Conservatives even though sometimes the Labour majority has been reduced to less than 5% (it was in 2010, but there was actually a 1.2% pro-Labour swing in 2015). The considerable UKIP vote is more likely to favour the Conservatives than Labour and even a mild recovery from the Liberal Democrats will finally unseat David Winnick, who has been Labour MP for this seat since 1979 (and also Labour MP for Croydon South 1966-70; note that the pre-1974 version of Croydon South is closer to the current Croydon Central). On current polling the result will be a foregone conclusion. Dead cert Conservative gain.
Wrexham. In Wales, Clwyd is the area fast changing from Labour to Conservative, and this will be first shown in next week's council elections in all likelihood. The Conservatives have been close in Wrexham before (missing it by just 424 votes in 1983) and Wrexham has been prospering in the modern era relatively well by Welsh standards. Labour is performing particularly badly in Wales, even compared to its traditional heartlands in the Midlands, and if the polls are remotely accurate, this seat will fall. Dead cert Conservative gain.
Wakefield. Wakefield, like Wrexham, was narrowly missed by the Conservatives in their 1983 landslide (by 360 votes) and is coming closer within their reach despite a pro-Labour swing of 1.3% last time. The Conservatives have been holding up well in the city of Wakefield itself despite UKIP's intervention and a relatively high Brexit vote will give them an easy win. Dead cert Conservative gain.
Copeland. Copeland is of course actually held by a Conservative MP now having been won by Trudy Harrison in the by-election of 23 February, and this happened due to the perceived anti-nuclear stance of Jeremy Corbyn. Little has changed since although as by-elections generally produce larger swings than in general elections, and Labour may recover in the coming weeks, Trudy is not assured of holding on in the by-election although it appears likely as much of rural Copeland has been trending away from Labour for some time. Likely Conservative by-election hold (general election gain).
Stoke-on-Trent South. Labour is still rather unpopular in Stoke-on-Trent even though they are no longer in control of the council, as memories still linger. Much of the pottery and glass-making has gone from Stoke-on-Trent and this is the most affluent of Stoke-on-Trent's three constituencies, with the turnout being the highest (although still considerably below average) and the unemployment rate the lowest. With a large UKIP vote to squeeze, the Conservatives will have no trouble here on current opinion polls. Dead cert Conservative gain.
Coventry North West. Metropolitan areas should be turning against Theresa May and her 'hard Brexit' mantra, but this varies from city to city and many cities are just as divided in that respect as small and large towns. As with many of these seats, the current Labour majority in Coventry North West is the lowest since 1983, and Geoffrey Robinson does not have a particularly good reputation amongst the left in Labour either due to his 'champagne socialist' lifestyle. Even though Coventry South is actually more marginal, it is in the wards of Coventry North West where the Conservatives have been making the most progress in recent years, and thus this seat could fall to them this year, although inner-city Labour voters are more likely to support Jeremy Corbyn's mantra. Probable Conservative gain.
Alyn & Deeside. The Conservatives are performing well in Clwyd but Alyn & Deeside, formerly known as East Flintshire (part of which in 1983 was moved to the current Delyn constituency) is holding up best for Labour out of the six and despite increasing Conservative strength in Flintshire, Labour could still hold on against the trend. However, the odds are not in Labour's favour at present by any means. Likely Conservative gain.
Bishop Auckland. There is considerable local strength in Bishop Auckland (at least by the standards of towns within Durham County Council) and only in 1983 was the Labour majority over the Conservatives lower than it is now. The Durham Council elections will be a key indicator as to whether the Conservatives can take the seat, even though some other pollsters have predicted it to be a nailed-on Conservative gain. The seat is turning against Labour but with the Conservatives unpopular in most of Durham, success is not guaranteed. Likely Conservative gain.
Mansfield. In 1987, Labour only held this seat by 56 votes due to the NUM being less than willing to campaign for Alan Meale, who is still Labour MP for this seat, and Labour is looking vulnerable. The Labour majority is higher than many of the seats listed above and whether they hold it will be a key factor in whether the Conservatives achieve a 3-figure landslide or not, and whether any Liberal Democrat recovery outside constituencies winnable for them occurs. One to watch for this election. 50/50 Labour hold/Conservative gain.
Workington. Workington is poorer than Copeland and also is less dependent on the Sellafield nuclear plant, and its Labour vote is more solid. Also, with the Conservatives needing to defend their by-election in Copeland, Workington will be easier for Labour to hold, but it is far from safe. Probable Labour hold.
Penistone & Stocksbridge. Whilst on paper it appears that the Conservatives may take it, this will likely not happen in practice due to the fact that within the constituency there is only Conservative strength in Penistone itself, and not Ecclesfield or Stocksbridge (there are no Conservative councillors in Sheffield at all nor any realistic prospect of there being any at the moment) which form half of the constituency. Even with tactical voting from UKIP, Labour will likely be returned as they have been in Penistone since 1935, as UKIP's fall could benefit Labour indirectly. Likely Labour hold.
Stoke-on-Trent North. Stoke-on-Trent North, like Stoke-on-Trent South, is turning against Labour but Ruth Smeeth was only elected two years ago and the 'incumbency bonus' should work in her favour. The Conservatives are not as strong in Stoke-on-Trent North, either, nor in Kidsgrove (part of the borough of Newcastle-under-Lyme), but a Labour loss cannot entirely be ruled out due to the question of where the UKIP votes will go (they will not all go to the Conservatives). Likely Labour hold.
Great Grimsby. Like in Wrexham and Wakefield, there have been several close shaves for Labour since they first won Great Grimsby in 1945. In 1959, they held on by just 101 votes in a straight fight with the Conservatives, achieved a majority of just 731 in 1983, and on his last successful election Austin Mitchell only held on by 714 votes; he retired in 2015 and was replaced by Melanie Onn. This is attributable to the small size of the seat, which will either be split in half or absorb the nearby coastal town of Cleethorpes in the next round of boundary changes. As Melanie did well to fend off both the Conservative and UKIP challenges (she achieved a 5.6% swing in Labour's favour) she has very good chances and Grimsby may benefit from Britain not having to comply with the Common Fisheries Policy once Britain leaves the EU. Likely Labour hold.
On current polling, of course, many other Labour seats will fall to the Conservatives as well, and every general election will spring a few surprises.