Circumstances meant that we moved out of London to the countryside (midlands) when DS was small, but I loved living there, and to this day miss the culture, and vibrancy, the energy, the public transport, and the astonishing richness of the things, often free, that you can do with children. And I do agree about the access to nature cycling is impossible around here, because of speeding cars on twisty roads who simply don't expect to see cyclists unless it's thirty MAMILs at the weekend, and though I take DS out on walks because being in nature is important to me, his classmates might as well be living in the middle of a concrete jungle rather than in a village surrounded by rolling countryside for all the time they spend out in it. Whereas in our grubby bit of zone 2, we had no outdoor space of our own, but a wonderful park directly across the street where we more or less lived, nearby nature trails and conservation areas and reservoirs where you could swim/sail, and were a short direct train ride from Hampstead Heath. And when the weather was poor, the museums alone were a fabulous resource and we were in them all the time. DS took his first step in the Great Court of the BM.
I think the only thing that would concern me about moving back to London is that we would have to choose a far more bourgeois area than our old grubby, vibrant neck of the woods, because I know there was a sgnificant gang problem with older kids and the schools were poor.
But yes, I think it can be a wonderful place to raise children.