Just checked in in AMS for AMS-LHR. Connecting to LHR-FRA-SFO on LH. Was told BA will not interline my checked luggage to LH at LHR since they're not OneWorld, and that I must enter the UK and pick up my bag. Lounge agent confirmed that this has always been the rule. Seems rather arcane to me?

I don't even know how to change terminals in LHR landside...