Most British websites work like this website suggests, in that they ask for the postcode first, then give you a dropdown of all the addresses at that postcode.

It works great, except my address is for some reason not in whatever databases these websites use. The building number is on the list, but not the individual flats. So I have to put in the postcode, choose something like "My address is not listed", then fill in the form manually. A few times it wasn't even offered as an option.