UK Towns Database Required

Where could I obtain a database of all the towns in the UK and their
related city.

For example:

Bilston would be related to Wolverhampton.

___

Well, ideally, where could I obtain a list of all towns and
their related Yellow Page area index (ie Wolverhampton area is 97
and so all towns in that area will come under 97) -

this data must be available somewhere.
.