I'd expect the Tory party to splinter the moment someone said "Europe".
We have broad parties full of people that don't like the views of many of the people they're associated with. Once it becomes possible to elect smaller parties I expect many of them to head for the hills.
Working out how many votes "The Conservatives" or "Labour" will get at in a proportional system is not likely to get you a very useful result.
(Been meaning to write this for a few days, and finally spurred to it when someone else posted about it, sadly flocked.)
Edit: fjm has unlocked their post. Worth reading - she's a better writer than me.