There are no terminals at AMS, merely different check-in halls and departure lounges. You'd probably come out of Arrivals 1 [possibly 2] from the UK Arrivals 3 or 4 from HKG and have to check-in with BA in Departures 3 - but it's a five minute walk to the other side of Schiphol Plaza and just up a floor.

Economy plus tickets on BA are definitely a good option, since you could then potentially change to the 0745 flight should everything go smoothly [or a later one if needed, assuming you can get online from the CX flight]. Personally I'd fly out of CWL if more convenient though, and the CX TPs would have to be absolutely crucial for me to take that over a decent J option.