Proportional Representation: No Longer the Enemy of the Establishment?

Proportional Representation has traditionally been a goal solely of smaller parties, ones which are unable to muster sufficient local support in single constituencies to win under FPTP - for example UKIP or the Greens. Large parties have traditionally been against it as the majoritorian nature of the current system favours them. It being possible to win huge majorities without a majority of votes.

Recent changes in the political landscape have thrown this into a tailspin - current projections show that Labour will win 42% (271) seats in the Commons with 33% of the vote. Likewise the Conservatives win a similar number of seats with a similar percentage. FPTP no longer provides them with a huge advantage.

The Conservatives would have the added benefit that it would gain UKIP something approaching 90 seats on current polling numbers, giving them a large block to form the basis of future coalitions (~305 seats, compared to 279 combined today) - which appear to be becoming the norm. They are likely to have far more in common than with other parties and would be natural allies.

Labour would likewise benefit in that it appears the collapse of its Scottish vote is permanent, and it would recapture a significant fraction of the lost seats as a result - after all the SNP would be reduced to half their projected seat count, almost all being transferred to Labour. It would be likely that the Greens and the Liberal Democrats (who would be driven from the Tory orbit by distaste for UKIP) would form a block with Labour, giving them a block of something around ~305 seats to work with, compared with 297 now.

In summary, the result of this reform would be the marginalisation of nationalist parties like the SNP and Plaid Cymru, which would lose a large number of seats to the fragmented unionist vote, whilst the other regional parties like the DUP or Sinn Fein, who do not compete against mainland parties, would be largely unaffected and thus more likely to support the reform.
Given that the large parties have considerably less to lose in the current climate  and the fat that the peculiarities of FPTP no longer guarantee majorities - perhaps the time is ripe for deployment of Proportional Representation?

An alternative member system could be introduced relatively cheaply by merging some constituencies (where they do not cover country or other social boundaries) to generate the space in the chamber for the Party List seats, as it would no longer be necessary to force constituencies to have completely equal sizes.