Phone International Directory Enquiries and ask them for the number of the Metropolitan Police in London. (Somewhat ridiculously, I can't find it for you because the only number on their website is 101, which is the nationwide phone number for non-urgent calls to the police, but which won't work from overseas). Call them and explain that you're extremely concerned for the welfare of a friend. They'll send someone round to check if he's OK or, if there's no answer, start making enquiries with his neighbours.
Or phone the Salvation Army, who will almost certainly try to help:
+44 20 7367 4500