Our main reason is still money, with the social problems second. Tbh, I don't think social stuff would be so much much of an issue when they get a bit older, as they will be able to go to things like brownies. We're losing our cheap and free mainstream and social under 5's activities, and the others are either too unstructered or very expensive for our tastes.
If school doesn't suit, HE is our next thing, and indeed we will be looking at it again regulary, as well as at any time when we change school for whatever reason.
It's all academic atm anyway - me and DH are working our little socks off atm, and it's all going to cushioning ourselves from being so near the edge, so things like HE can be an option for us.
I did set up my own group. I met a couple of lovely people, who I am still good friends with, but they were still part of the same "gang" - there are only so many home educators with kids under seven in each area, and most of the other groups were out in the countryside or four buses away.
It boils down to me not being strong enough to fight on this issue. If I get the slightest whiff of school not working, I'll fight tooth and nail to HE, but for now, and I'm not proud of it at all, school seems to be ok for my kids so I don't have the energy to fight. I'm fighting on other issues for the moment.