In reply to Jay_Kay:
One way would be MPs without constituency. So you would have 650 MPs voted in. Then you would add MPs to get propotional. So very simply, with 5 consituencies:
1: Lab 50, Cons 30, Lib 20
1: Lab 50, Cons 30, Lib 20
1: Lab 30, Cons 50, Lib 20
1: Lab 35, Cons 45, Lib 20
1: Lab 35, Cons 45, Lib 20
First Past Post:
Lab 200(40%), Cons 200(40%), Lib 100 (20%)
Lab 2, Cons 3, Lib 0
PR adjustment:
You add 2 labour MP, 1 cons MP, 2 lib MP picking high polling but unsuccessful candidates.
Total 10 MPs, the party representation is proportional to national vote, but each constituency has their own MP.
Unfortunately you end up with loads of MPs. So you need to make boundaries bigger.