As Heathfield indicates, it's illegal to provide 'reverse-searchable' telephone directories, either online or on CD, in the UK.
As has been suggested, you could try the electoral roll. However, this assumes that you know the surname of the relative but I have to assume that you don't. (Otherwise, assuming that the number is not ex-directory, an ordinary telephone directory would do).
There are two additional problems. Firstly, well over a half of all UK residential phone numbers are now ex-directory. Secondly, a large number of people (possibly the majority of the adult population) tick the box on the electoral registration form to have their details removed from the 'edited register'. (This is the only version of the electoral roll which is available to the general public, either online or on CD). So, even if you know the surname, you still won't be able to find the address.
It looks as if you might have to phone the number or, if this presents difficulties, ask a third person to act as an intermediary and call on your behalf.
Chris