If you're less than 24 hrs at LHR and on the same ticket you are not hit by APD - but you're still hit with BA's extortion money ("carrier imposed charges")

Flying OW in Europe also means AB (TXL, DUS), AY and IB (MAD) so depending on your final destination you might want to consider those. Getting out of Europe also means AA/CX/JL/MH/QR and QF(?) They have non-LHR departures from the UK with no/low APD and of course also non-UK departures.

Do remember, that OW redemptions are often cheaper than partner redemptions - flying 2 OW partners already puts you in the OW redemption pricing, flying only one is partner redemption. Just check if the feeder can be on another airline than the longhaul.