Even though Scotland voted to stay in the United Kingdom, the strength of the Yes vote in areas that are normally solidly Labour, combined with increasing resentment of the three major parties (Labour, Liberal Democrat and Conservative) and a need for major change and devolution across the whole of the United Kingdom, gives hope for the various centre-left to left-wing nationalist groups within the United Kingdom. The groups in question are the SNP, Plaid Cymru, and Mebyon Kernow. (It may also be worth talking with Yorkshire First, as their vote share was better than average in North Yorkshire in this year's European elections compared to other parts of Yorkshire and the Humber).
Given that the Green Party is unlikely to field a full slate of candidates across England and Wales (the Scottish Green Party may not field a full slate either) it would be wise, given how divided the right is in the UK (there are four right wing parties of significance) and that the Green Party is in the same European group (G/EFA) as the SNP and Plaid Cymru, for some cooperation to occur at the 2015 general election. For example, I believe the Scottish Greens should not stand against the 6 sitting SNP MPs (but should stand everywhere else if possible), and that Wales Green Party should not stand in the 3 seats Plaid Cymru currently holds (or Ynys Mon, Plaid Cymru's top target).
I believe this can be a useful strategy for the next general election, given that it will help provide all of the British electorate with a progressive alternative even if not everyone will be able to vote Green in 2015, and help ensure further devolution in all regions of the UK when needed. There are several seats in the UK where the potential for a win by a nationalist group (Plaid Cymru/SNP) is reasonably high; these include Ynys Mon and Ceredigion in Wales, (and maybe Aberconwy on a very good election year), and Ochill & South Pershire and Gordon in Scotland.
Together, we can all help provide a new vision for Britain, one of hope, fairness, equality, sustainability and peace.