I know thank answerbankers are great but i think it's highly unlikely you are going to get someone who
1) has raised a child in ZA AND
2)has also raised a child in England AND
3) can remember the cost of things
Maybe someone will come along and prove me wrong.
I know you havent asked for advice, and you really want to look into things before you move, but my advice would be that you just have to suck it and see. Gym member ships, commuting costs, flat rentals and child care costs are all going to be completely dependant on idividual circumstances and preferences so you really wont have any meaningful answers!