Originally Posted by
teledude
Yes that will work fine
It will work but it's not as simple as that.
Upon entry to BA and CX lounges, there is the possibility that they may change the FF number in your booking. Therefore make sure after you've been in the lounge that you ask them to check which FF number is against your boarding pass and make sure it is the correct one.
Assuming you are flying BA from T5, after entering the lounge, ask the lounge services desk inside to check the FF number.
If you are departing HKG and flying BA and you enter a CX lounge, showing your CX card for entry should be fine, as CX doesn't have access to change the FF number against your booking.
However if you are flying CX from either HKG or LHR, make sure after entry (usually on your way out or mid visit), that the FF number is correct. The reason I say not to do it during entry is that the attendant will probably get confused what you are asking.