It is not just a terminal station (There are two or more different stations that can be reached from it, on separate lines.)
The above is an extract from the rules. My understanding of this rule is that the first station has to be on a different line totally from the other, not just having platforms on one route only. In my view the following London terminals cannot be used as starting points:
Waterloo
Charing Cross
Cannon Street
Fenchurch Street
Liverpool Street
Moorgate
Kings Cross
Euston
The following I consider to be allowable:
Marylebone (Wembley Stadium/Harrow-on-the-Hill)
Paddington (Acton Main Line/South Ruislip)
One I'm not sure on:
Victoria (Battersea Park/Brixton) as this could be a Liverpool Street scenario, but Baker's atlas suggests not.
St Pancras, Blackfriars, London Bridge etc. are all through stations in some way.