Note that if you booked AMS-HKG//BKK-AMS on ba.com, you will be priced in EUR and a UK Amex will charge you (IIRC) 2.75% as a FX fee. That completely dwarfs the cashback from topcashback, so you may want to factor this into your planning.

If you phone BA to book, you can be charged in GBP so you don't pay the FX fee. However, there will typically be a £10 service fee plus £5 credit card fee per person for booking this way. If you can do the upgrade using Avios at the same time as booking the ticket, AIUI the £5 credit card fee won't be charged.