If you stop off in AMS and don't break your ticketed journey in London by more than 24 hours then you should at least avoid APD. But if you make the AMS trip on effectively another ticket, it will look like a LHR stopover and APD would be payable on the first ticket.

It looks quite complex to me, and it is the sort of journey I give to my travel agent to sort out. And I don't mean an online travel agent! Unless you get a really clued up BA customer service representative you may indeed be paying extra.