It worked in the 50s BECAUSE the garden cities were designed and built for people escaping war-ravaged London. What the OP is saying is that there doesn't appear to be an equivalent being designed and built now.
Also, as someone who has escaped London's expensive housing to a different city, we have had to think about moving back to the southEast because of a lack of working opportunities here. However, even to live in a not-terribly nice bit of Hertfordshire (compared to where we live now) we would have to pay an equivalent of a second mortgage for one annual commuter pass. So if we were both working in London, we would be paying an equivalent of 3 mortgages to live inside or around the M25.
Plus, unlike when we lived in central London, it would be harder to exist without a car, plus child care would be expensive as we'd have to add in an hour either side of our working days to allow for commuting time.
For us, cheaper to live outside London in a nice area where we have lots of support and informal childcare help, and to work more sporadically.
London has this huge gravitational field and it pushes prices up for miles and miles around, you have to get well out of town to get away from it, and the work opportunities get less and less....