I think the point (see GP's other comment 'Philadelphia was the highlight of my trip to Washington D.C.') is that it's not part of a trip to London, your trip is to the UK, or England, you stayed in London, and visited elsewhere too.

A lot of foreigners seem to conflate London with the entire country in a way that doesn't seem to happen for other countries (you don't 'go to Paris' and actually stay in Marseilles) and some people outside London especially understandably don't like it.

Not that it's an unreachable distance away for a trip based in London.