As far as I know, Kings Lynn, Peterborough, Bedford, and Cardiff are not in London. Nor Brighton, Dover or Southend.
You can model most Southern Routes, LTS, Great Eastern, Great Northern, Midland and Great Western. No other unit type offers this, so that's half the British population covered!
In time these units will spread North, but as tested on the WCML from Crewe (not in London), anyone could model them.
PS - Southern class 401,402,403,410,411,412,414,416,419,423,450,466,438,491 units have sold well enough, so clearly plenty of Southern DC modellers out there!