I'm certainly not an expert on the subject but I think the issue is to do with a lack of BA surcharges ex-HKG.

I am assuming that you are searching for one-way on the website calculator, and so that will generate an ex-HKG with few (no?) surcharges.
However as soon as you want to book an ex-UK return then I understand that the UK surcharges will apply to both the outbound and inbound legs.
The way around this would be to book 2 one-way flights, but in your situation that doesn't help much as you'd then only be able to apply the 241 to either the outbound or the inbound, not both.

I'm not sure there is a way around this unfortunately.