Seconding Mark comments, I've just done a quick look up on a Gig Les circuit (backhaul) from Godalming to Telehouse, and its circa 30K pa. From Cardiff with the same carrier, its well over 100K pa..
I can see their logic. They also take into account the average line length in the area as they will not deliver to >5km lines which also may have an affect..
The other aspect is that in a large area such as Cardiff, I understand they dual attach the central exchange and spur off to surrounding areas. While with Surrey this is easy, there are less major conurbations in short haul of Cardiff to allow them to do so.
There are obvioulsy other issues such as whether there is physical space in the exchange building and security restrictions in some exchanges. One we looked at had a high contingent of government data, and BT seem to take more care of it than HMRC and others and required partitioning within the building to allow third party kit in, which we would have had to pay for....