If it is likely that they have married, died or had a child this will be recorded in the public records office in Roseberry Avenue EC1. The address is recorded at the time. You can also look up old telephone directories for all parts of the country at the City of London library and keep looking for entries for your lost person at their original address until the entries stop, which gives you a timeframe to start looking at the public records. It's quite time consuming and you can hire someone to do it for you, you will find adverts in the records office.