Stefano,
you might find this site useful: http://www.mortgagetalk.co.uk/buying...alculator.aspx.
You mention you have two pre-school children. In England education is compulsory from age 5, but many local schools will start taking children as young as 3 into nursery classes for 1/2 a day. I suggest you contact the Local Education Authority for the areas you are looking at to find out what is on offer. Of course, if you will be wealthy enough to take the private education route I'm sure you can find professional bodies ready to help you out.
As other's have indicated, house prices can vary enormously, often connected to how 'good' the local schools are.