Hi again
I live in Tooting and there are loads of us around. Often you'll find that people are at work 3 days a week so you'll have 2 days with your mat leave friends around anyway for playdates etc. Otherwise, if you go to places that other mums are - toddler groups, classes, gym etc - you'll make friends if you are happy to chat to people and they might be on their mat leave for now, but they will be around for 9 months or so. It is a little cyclical, but you will make good friends and then you'll find your, say NCT friends, will be off again with number 2 and so on.
I volunteer with the Wimbledon and Wandsworth NCT and they are always looking for volunteers. The toddler group is through my church. There are lots of local mums on Twitter - the Tooting ones had a meet up last week. And there are some cheap playgroups around here that from about 2 years you can get morning sessions for a couple of hours at under £10, so not breaking the bank and some time to yourself, and some time to accustom your child to that kind of environment if you want to (I was thinking I could perhaps get a day or two paid freelance work spread across the week if I went down that route).
I guess I've never had to think about going back to work - in fact I was already working as a volunteer for about 18 months before we had our son as I had some visa issues (I'm Aussie) so I didn't have a job to return to but this is my experience and while I do keep an eye on the job market, I don't think I could go into full time work until our kids are in school (and then you have the whole...how do you pick them up thing to think through!)
Good luck with your decision x