Regarding stations in London being in the centre, I would like to mention St Paul's although it is part of the London Underground Central Line.
When I was investigating driving routes for a friend of mine a number of years back, I had input "Glasgow" and "London" on the AA (Automobile Association, not Alcoholics Anonymous) for directions. Being as these were generic terms, both ends used as the reference point were the former GPO buildings in George Square and St Paul's respectively.
A mainline station I am unsure as to whether it is central for the town is Weston-Super-Mare. Although it could be said that it is quite a hike to see the sea there
For major cities such as Birmingham, Liverpool, Manchester, etc, a rule of thumb that could be used as to determine whether it is a central location or not is to check if the postcode of the station falls between 1 to 5. For example, Glasgow Central, High Street, and Argyle Street are G1, Anderston Cross, Charing Cross, and Glasgow Queen Street are G2, and Bellgrove is G4 (although borderline East Central, as continuing east of Bellgrove you would switch to G31).