Caroline Lucas, the Green Party's only MP, has been broaching the subject of Progressive Alliances again publicly, even though Labour leader Jeremy Corbyn has given no indication that he will change his mind, and nor has Liberal Democrats leader Tim Farron shown any real interest.
A look at recent histories of progressive alliances, not just in Britain but also in Europe, shows that the idea is not all it is cracked up to be:
1992, Green Party-Plaid Cymru in Wales: This was primarily responsible for electing Plaid Cymru MP Cynog Dafis in Ceredigion (otherwise a reliable Liberal/Liberal Democrat seat) but otherwise did not give much help to either party given that it only occurred in a few other Welsh constituencies and did not substantially increase Plaid Cymru's vote from 1987. It collapsed by 1995 and has never been revived since even though it could have been successful in the long-term.
1996-2007, Olive Tree in Italy: A very wide tree this was as well, comprising the Democratic Party of the Left, the Italian People's Party, Italian Renewal, Federation of the Greens, Italian Socialists, and Democratic Union. It won 45% of the Chamber of Deputies seats and nearly 50% of the Senate seats in 1996, but initial optimism faded in just a few years, in a similar manner to Tony Blair's Labour government of the UK (which was in fact anything but progressive all along). The most radical two parties, the Communist Refoundation Party and the Federation of the Greens, left the alliance the soonest, The remaining Olive Tree parties formed the Democratic Party, in reality just as in line with neoliberalism as most social democratic parties in Europe.
2003, GroenLinks-SP (and GroenLinks itself in general, which was formed out of several different parties with different radical backgrounds): GroenLinks, the Netherlands' largest Green Party, formed an alliance or lijstverbinding with the Socialist Party, the largest clearly socialist party in the Netherlands for the 2003 elections. However, neither party made any headway and GroenLinks actually lost two seats. GroenLinks has not always been an easy alliance and goes from strength to strength, with the animal rights party PvdD taking some supporters of GL, although it is currently polling well again.
2012 French legislative election: Every deputy of the EELV (the French Green Party) who was successfully elected received endorsement from Francois Hollande's Parti Socialiste (PS), as were some unsuccessful candidates. However, with Francois' (and PS')plummeting approval ratings, his failure to deliver on key issues like fair taxation, and his increasingly authoritarian attitude means that EELV will probably not welcome such endorsements next time.
2014, MSZP/DK/PM/Egyutt-2014 in Hungary: Formed in response to Fidesz' neoconservative regime and gerrymandering of the electoral system (by lowering the number of MPs to 199 from 386 and by having more SMC seats than list seats) in the hope it would be enough to overcome Fidesz' supermajority. However, due to the bad reputation of its leading figures, such as Gordon Bajnai and Ferenc Gyurscany, as well as corruption scandals within the Hungarian Socialist Party, it barely made a real dent in Fidesz' supermajority. The Unity coalition dissolved soon afterwards.
2014, Internet Party-MANA in New Zealand: The MANA Party, a progressive Maori interests party in New Zealand/Aotearoa, formed a pact with the online rights Internet Party which was founded by Megaupload founder Kim Dotcom (real name Kim Schmitz). Kim's subsequent arrests over illegal downloading and the international furore this caused was a key factor in Hone Harawira, MANA's only MP, losing his seat; the coalition won just 1.74% of the vote and gained no seats at all. This coalition has since split up.
2015, Democratic Left/Polish Labour Party/Green Party in Poland: Called United Left and led by former Polish PM Leszek Miller, whose tenure was marked by 'Rywingate'. He is still a relatively unpopular figure in Polish politics, and the general shift to the right in Poland caused this alliance to poll just 7.55%, meaning that it lost all parliamentary representation (The normal threshold is 5% but for coalitions it is instead 8%), partly due to the more left-wing Razem Party winning support from lapsed DL voters. Needless to say, it dissolved within months.
2015, Croatian Labourists et al. in Croatia: The Croatian Labour Party joined up with centre-left (in reality soft social democratic and neoliberal) parties in the Croatia is Growing coalition; the perceived betrayal caused half of its six sitting MPs to defect to Croatia's main Green Party, ORaH. It retained 3 seats in the Sabor in spite of this opportunistic collaboration, although it has since abandoned any alliance with Croatia's Social Democrats.
2015 (both January and September), SYRIZA-Ecologist Greens: This coalition helped the Ecologist Greens, Greece's largest Green Party, gain representation in the Hellenic Parliament for the first time (they came within 0.1% of winning seats in May 2012 standing on their own). They currently have 2 MPs in Greece and Giannis Tsironis serves as Environment Minister.