The 2002 census from the Office for National Statistics showed London (comprising Inner and Outer London) to have a population of 7, 172,091 people. That's a lot of people living in a densely populated area, so finding one person is challenging.
Make sure you have the full name of the person for whom you are searching. Check that it's spelt correctly. For example, clarify whether it is Davies, Davy or Davis. Common spelling errors will put you off-track straight away. Ideally, a name comprises first name, middle name, and surname. If the person you're trying to locate is female, it's helpful to know their surname before and after marriage.
Obtain the person's last known address(es). Partial addresses, such as London street names, names of houses they lived in or general areas (e.g. London boroughs, like Walthamstow or Haringey) where they resided, can be useful. Don't assume incomplete information is useless; an online search using surname plus borough or street name can bring results. If you are unsure of spellings, but know what the areas sound like, write this down. If you visit, local people will help you match correct names.
Begin online research into the person while you are still at home. It's easier using online resources than visiting London and searching "on the ground". Using search services at home might also be easier, since some paid services require alternative payment methods that may be unavailable once you are travelling. It is useful to have an active VISA card and a PayPal account.
Start with AT&T service anywho.com and browse the UK database. Or visit the online UK telephone book, 192.com and open an account. Deposit funds to obtain search credits. If you are not a UK resident, you'll need a PayPal account. Using credits, search against London residents' names and addresses. 192.com holds information on all UK registered voters, from 2002 to 2009 as well as those on the UK telephone system. If the person for whom you're looking is a director of a business you can search under their business trading name.
If you cannot find the person on 192.com, ihe may have requested that his details be removed for personal reasons. If the person has an unusual surname, and the search has not revealed your friend but has revealed other people with that same name who are also in the London area, you might try calling some of those to see if they are related.
Think of interests and hobbies your friend enjoyed, and search for her against those hobbies in the London area. If she enjoyed photography, run a Google search against her name and London photographic societies. Also, use the online social networking resources that aren't geographically restricted, such as facebook.com, linkedin.com, pipl.com.
Visit your friend's last known address and enquire of neighbours; they may know when he moved, and where to.
Consider whether it is appropriate to commission a professional search agency to carry out a thorough search.
When you make house calls, be careful not to cause anxiety. Do not knock on doors after dusk. It may be more courteous to leave a note and allow people to contact you. If you feel someone knows your friend but prefers not to give you information, do not press the point or visit again.