When we first started ttc (sadly some years ago now and no actual living baby as yet), we looked at moving out of London as I sort of felt that so many of our colleagues couldn't be wrong.
But, basically, if you want a place that's a reasonable commute (an hour door to door is our max), it is more expensive than where we live on the border of zone 1/2, especially when you factor in season ticket costs. Important for us, too, was amount of time spent at home with our as yet hypothetical children - I don't see the point in having a nice five bedroom house if the kids are asleep when you get home and you barely see them during the week.
One of my former colleagues has a very high flying career, as does her husband, and they've gone for a two bedroom flat within 5 mins walk of their offices. It means their kids are sharing a room and they have no outside space but, on the other hand, they aren't spending 10+ hours a week of their lives on trains but with their kids instead.
We can't afford a place that close to our offices but, essentially, that's the same reasoning as us. I think moving out of London and commuting only really works if one of you either has a job that they can do locally (e.g. teacher, doctor, nurse) or is willing to stay at home with the kids. Both of us have chosen a career that we really need to be in London for and neither of us wants to give it up. So, yeah, it's stupidly expensive but we have our reasons for staying put...