how can i find the first name of someone in uk if i know the last naem
and the address?

Hi, if this white pages site does the trick, I'll repost this comment
as an answer. But it will probably depend how common your last name
is. Smith? Then the city won't be enough.
Good luck.
This is an infospace international phone directory:

http://www.infospace.com/_1_175054633__info/fill?NF=intldb/intl-people.html&QLN=TRUE&QFN=TRUE&QCY=TRUE&Country=United+Kingdom&KDB=intl-uk&OTMPL=/intldb/intl-out3.html&ESCEA=&QO=UK
LMNOP-ga

When stalking, I've had very good results from http://www.192.com

When I last used it you have to register, and they have an irritating
credit system, but it often gives the first name if it finds who
you're looking for on the electoral roll.

If this site helps, then I'll be glad to post it as an answer.