Originally Posted by BA6501

Or simply HKG-LHR and AMS-HKG, over the phone.

Originally Posted by marcolau317

He is flying FROM HKG to LHR/LON, and then return from London. Therefore, it makes a lot of sense to book a round-trip ex-HKG instead of 2 singles, as it will lead to extra YQ on the second one-way ticket.
Do note: There is no upgrade voucher or 2-4-1 for Ex-HKG bookings (or in HK at all). We don't have such thing here... =(
The actual answer is: it all depends on how you intend to book.

If you use Avios to book EITHER BA Only, CX+BA or QR+BA, then normal rules apply:
No refund of the ticket after first departure.
Changes is permitted till 24 hours before the flight (the changing one, not the first one is due to depart)
Changed date MUST be within 1 year from the first flight
Subject to Avios redemption availability at the time of changing
Airline changes: MUST maintain BA+ max 1 OW (so as to NOT trigger the multi-carrier award chart)
Departure point change: Change shall not exceed to the booked distance band
Change Fees required: GBP 35 or HKD 427 + Additional Tax + Additional Avios (If moved from off-peak to peak flights)

If you want to book BA+CX+QR or CX+QR, do note multi-carrier award chart WILL apply in this case, which means in additional to the rules above:
The total distance must remains within the same distance band
Must remain at least 2 non-BA OW
Maintaining maximum of 8 segments
If the booking involve multi-class, the largest proportion of distance should be in the cabin you paid for (or additional Avios applies)

Unless you book standard round-trip, most probably you have to call BA to complete the changes.
Do note CX reduced/hide the availability after 22 Jun Asia Miles changes, and it may involve married segment concepts.