My guess is that I was charged for only PE tax and surcharges £254, and in reality I should be charged for CW £650. Maybe I can verify via ITAMATRIX to find out the actual difference.
I think my point is I booked this tkt online with avios and guf1 at the same time, that’s why it was miscalculated. And if you book a return HKG-LHR-LHR with avios and guf1/2, the tax & surcharges would be less than the actual amount an BA agent would tell you over the phone.