I've not tried the pool in Streatham... you could give that a go?
Clapham Manor Pool is ok - although I think it's a bit spermy.
Dulwich College pool is gorgeous but I think you need to be a member of their gym (but it's worth checking that out).
East Dulwich is another one I've not tried...
Tooting is open year round but you need to become a member which was around £100 p/a when I joined (briefly) a few years ago.
The Lido is still open for morning swims btw - until the end of next week.
Camberwell is v. cheap but rusty. I rather like it though. I like that old fashioned pool vibe - Lewisham is similar in that respect, as is the pool south of Norwood Junction.
If you join the Lido gym you can use East Dulwich and E&C pools and the gyms at all Fusion centres as well as the lido all next summer. (but if it really is only the pool you want, and not the gym, it might not be worthwhile).
I love swimming but it you and I are in a tricky situation right now - the Rec situation is disgraceful. I might join a particular tri club in order to get lessons in one of the local school pools.