I think Wales is probably a bit drastic to be honest, OP. It’s ok for him, he grew up there. Yes life is undeniably cheaper, but you’ll be earning less so not necessarily better off in the longer-term.
I know you haven’t started TTC yet, but think about schools. Where are there good primaries? In Wales, would they not be doing compulsory Welsh and a different curriculum?
Could you not just compromise and move a bit further out than Purley? Sussex and Kent have some lovely parts that feel quite rural. The South Downs are beautiful and a lot less rainy than Wales, it has to be said.
Think about what what your kids will be doing / hanging out with as teens. SE London obviously has its risks. But there’s not much for teens in small Welsh towns and this can generate a lot of antisocial behaviour. There is a lot of deprivation in Wales - it’s not all idyllic obviously.
If you stay in London or the suburbs, any house you eventually buy will only go up in price at a different rate to elsewhere and this is the legacy you’ll be leaving for your kids one day. Plus, when they get jobs (I know this seems a long way off), chances are the opportunities will be In London. It will be easier to support them. Once you get off the London property market, it’s hard to get back on.
You won’t need a house straight off with a baby. Buy a flat in the recession. It can only go up in price and house prices will go up again at some point. Then use the money you’ve made to buy a house. You don’t get the price accelerations anywhere else really (we’ll not to the same extent).