The North East (Sunderland, Newcastle) sprung to my mind as being the most likely ... and indeed checking online they seem to be about the lowest cost housing in England or Wales. A very long way from London, though ... read carefully the posts of other expats in that area, get in touch with them. You might feel isolated.
Other places I came up with .. Swansea, Skelmersdale, Workington, Carlisle ... suggestion - get hold of a map and try a few places in
http://www.rightmove.co.uk, asking for a good area around the place you put in.
Anywhere towards the South East will probably be way above the prices you have quoted. For example, we live 100 miles from London, and 3 bedroom accom.s around here start at about twice the price you quoted.
Will you be visiting to look at the areas? Suggest you do - you might hit on something that's a real gem of an area for you, even if it's not what others might like. Also watch school catchment areas and (seeing as you're running a web business) broadband availability. Not yet universal (and won't be universal for quite a time), you can check the postcode at the BT site, Telewest site and NTL site to look at the options that are or are not open on this.