Minor point - and I don't think I'm being pedantic, because I think this could potentially be meaningful in Air China's rules: you probably do not have a stopover in Beijing. It's likely a connecting flight under the LHR-BKK tariff.

If you did have an actual stopover, e.g., LHR-PEK, stop, PEK-BKK, BKK-LHR (whether connecting back through PEK or not), then you might have more options. You'd need to read the fare rules to understand how it was priced and what you could cancel (if anything), keeping the remaining two parts intact.

Some airlines are more generous than others with stopovers, often because they want to encourage a visit to their hub city, but you usually need to fly all the segments. All of this discussion assumes you're on a nonrefundable coach ticket with the typical associated fare rules.

Isn't Bangkok more interesting than Disneyland anyway?