Calcot manor near Tetbury is great, has spa and good food. Been there a couple of times now, most recently 2 weeks ago. Our daughter was happy painting in the crèche for an hour whilst we used the spa. In the evenings we ate in the restaurant - they have a listening service, or can arrange a babysitter to come to your room. We did the latter when our daughter 8-months old and the former the other week (being slightly more confident parents these days
).
They do only let children in the pool at certain times though, but it wasn't that busy. We also borrowed a bike with a baby trailer and cycled a few miles around the grounds.
Got back to Balham in under two hours (travelling at Sunday lunchtime).
Enjoy and I hope to see some other suggestions on here too. X