Meggles info is a good start, obiously the figures will vary to an extent depending on the house, area, solicitors used, how much deposit you have etc etc.
School year is relatively easy to find out, email some of the schools in the area - but generally UK school year is 1st/2nd week of september to mid/late july
All the 'disposable' income costs you mention will vary, childcare costs, meals out, drinks out, car home insurance all will vary, so best to ring around various insurance companies to get an idea of your particular situation. The links meggles gave are great to start your research.
As an aside, Devon is one of those counties that has experienced a relatively rapid increase in property prices. Alot of people fed up of the constant rat race in London have moved to devon and accordingly prices have risen. In conjuction, as it's a scenic type county, that attractiion has added to values - on top of that, many of the newcomers have been retiree's and the surplus in wealth by moving to Devon means there's been a corresponding apparent 'increase' in desiribility over the last few years.
Good luck !
DtM! West London & Slough UK!