Hi pmy3
If you can't find them on the online electoral registers, it means that they may have 'opted out', they are still eligible to vote but don't appear on the lists that are sold to the likes of 192.com and marketing companies.
You mention you are in London. I suggest you take a trip to the British Library at St Pancras and register as a reader. If you know any last address, you can order up the full registers there and check to see who, over the age of 18, is living there now. They are still arranged by street address, not name.
Dawn