Ordnance Survey have released quite a lot of data for free use (http://www.ordnancesurvey.co.uk/oswebsite/opendata/).
The most interesting sounding data (meridian) is a 2Gb shape file, so I chickened out of trying to do anything with that tonight.
Instead I downloaded the 50k Gazetteer and have put it into a postgresql database so it can be queried to look for things.
Work in progress is at http://www.maps2.webhop.net/openos.
It sort of works (just not very useful yet - you can do http://maps2.webhop.net/openos/gaz/www/doSearch.php, and it gives you a list of all of the items in the database containing the string 'hartlepool' - just need to transfer them back to the browser and plot them on a map now....