I think that could definitely be an option - they'll likely have a large majority this time round and do no incentive to change the system but, as the margin erodes over the years and they have to compromise more with other parties, this will likely be on the table.
Another factor is Scotland. It will eventually go and that would likely mean Labour would only ever win with a massive turnout in their favour. I also think there were concerns about boundary changed helping the Tories. That would mean the only way to keep the Tories out would be PR and Labour, less likely to benefit from FPTP, could decide it's a better option for them.
I'm not so sure about Scotland. There have only been three elections since 1945 where Scotland was the deciding factor in the result, and one of those delivered the Conservative/LD coalition.