In the past AMS was cheap, also in general fares from Europe to UK are half the price of UK to Europe fares (at least for J). MA often has unadvertised very cheap J fares LGW-BUD but you have to phone them or they offer an upsell when you buy Y (so not that convenient, but can be 25% of the BA LHR-BUD price). AY can also be cheap in the winter as long as you continue past HEL to somewhere unpopular.