It's good to see that the GB£ UK261 amounts are listed with an applicability matrix. There is no excuse for BA trying to pay €, save in a case where a passenger is also covered by EU261 (as distinct from UK261) and claims € amounts under EU261.
The below information is only relevant to flights covered by UK law and applies to all such flights, however you booked the flight. To be covered, your flight must either:
depart from an airport in the UK on any airline
arrive at an airport in the UK on an EU or UK airline
arrive at an airport in the EU on a UK airline