In the USA, we have cabotage laws that prevent foreign carriers from taking pax or cargo from one US point to another, whether they be ships or airplanes.
A BA flight from LHR to JFK many proceed to SFO, for instance, and discharge it's thru pax there. It may not however pick up pax at JFK, carry them to SFO for a fare, and discharge them there.
I believe such laws exist in other European countries. For instance, TWA operated JFK to MAD, then on to Malaga. We could not board paying pax in MAD for Malaga.
I support such laws, and I'm surprized the UK does not have similar ones.