This ignores the effect a PR system will have over time on parties themselves. There will be far less broad church in each party, there will obviously be some but within a much narrower "ideological framework."
Taking the Labour Party as an example, there will absolutely be no need for 4th Internationalist Trotskyite entryists to ever join Labour. They can all quite happily sit in their nice little TUSC environment because, if they campaign across the country and get enough of the votes they will get some effective representation in their own right and a much better opportunity to further their aims. As it stands now that won't happen, TUSC will always be a minority with no representation etc, they are left with infiltration of another party as their only opportunity to effect change. With their own possible representation, they may even get to a stage where they can effect some meaningful change.
PR will naturally narrow the ideological imprint of all parties. The Tory Party will be no different. Those that left during the Brexit debates, the likes of Anna Soubry et al, formed a new party. It was always doomed to failure because of FPTP. FPTP absolutely prevents the formation of effective new parties with new fresh ideas