Absolutely.
Your case is identical to me and my Australian savings and local Amex credit and Visa debit card.

HSBC promos - you can just think of it as a 8% cash rebate equiv (in miles).
Fubon - about 3%.

Exchange rate is also a factor - if you think GBP is low then spending in HKD means indirectly buying GBP. You can run down your GBP savings when rates are high.

Last Easter back down under I maxed the HSBC overseas promo in place at that time. Given it's my first trip back after Australia's changed to a Chip and PIN jurisdiction I've been getting very weird looks when the terminals start spitting out signature slips and asking cashiers/waiters to verify my signature.