My advice, be really realistic about what you can afford, two kids at a prep in London is around £16k per annum and that not taking into account all the peripheral ancillary costs, trips in y3 £300-500.
Uniform, growth spurts, summer and winter, loss and damage - it all mounts up.
Transport to and from school, especially if its further than you would normally travel/walk.
Before and after school care (especially if your both working).
This is the breakdown for last year year 1 & year 3.
Basic school fee costs £16500
Uniform costs (this also includes a change in uniform style mid year). Coats, hats, scarves, ties, blouses, tabbards, summer dress, socks, tights, trainers, plimpsoles, Jogging top and bottoms, netball skirt, swiming costume, rucksacks, book bags, gym bags, swimming bags, Jumpers and a few other bits and bobs. £575.
Additional stuff for classroom Dictionary, English to French dictionary, English to Spanish dictionary, pencils, colouring pencils, glue sticks aprons (art & cookery) £200
breakfast club and after school club £900
Half term and school holiday clubs £2500
year 3 week away trip £450
trips to zoos, museums, galleries and theatre £150.
There is probably a lot else I've forgotten. Also this is year on year plus inflation for the next n number of years