We are in zone 3 and really like it.
We do have a garden though the kids don't use it much - they are 6 and 3 and much prefer to go to a park.
Why we like it now:
Variety of things to do - before anyone starts up, I am not saying nothing happens outside of London but population density does mean there is more to do - both for the kids and for us
Childcare options - when DS's nursery closed down, there were several options. If we want a babysitter, there are plenty of ways to get one. If we need a holiday club, there are dozens in the immediate area. Somewhere like Cheltenham there will be options but not as many.
Diversity. I am not white and neither are my kids and I feel that they don't stick out here in the way they would in some other areas of the UK. Again, preempting the inevitable, I know that there are cities with more diversity like Birmingham or Leicester but it's one draw for me.
Don't need a car - neither of us like driving.
When they are teens, I am looking forward to not having to drive them everywhere. When they are young adults, there is a higher chance they will stay local as there are so many jobs in London.
I am no wedded to staying in London - I would consider other cities in the UK, especially Birmingham, Manchester and Sheffield - but I personally prefer a big city to somewhere like Cheltenham.
You could look at Birmingham and your DH commuting from there - there is a lot more going on there and you could probably afford a nice house.