i think what u are after is Ordanance survey maps. i'm afriad you can't really get these free as they are copy right protected. Have a look here : http://www.ordnancesurvey.co.uk/oswebsite/
These maps are usually required when u submit a planning permission application to ure council.
On second thoughts there is one way of getting these maps for free, go to ure local library - the main one, ask to look at Ordanance survey maps, choose the area and find which section you need then see if you are able to take it to the photocopier in library to get a copy of it. Like i said they are copy right protected so they may not let you take a copy of them.