It's a two stage process:
1. Go to www.bt.com and click on the big blue At Home rectangle. At the next screen go right down to the bottom and click on Find a Number. Now click on UK codes and clear the box headed Region or code. Enter the area code for your friend's number (the first five digits beginning with zero) and this will take you to the town where he lives.
2. Now go to www.192.com and enter his name and town in the appropriate boxes. Press Search and you might be lucky.
The results are in two sections; one showing phone numbers on the right hand side. The addresses for these phone numbers are on the right hand side.
If he is ex-directory, you'll not get his address anyway.