On a long long-haul flight such as LON-HKG, then you really do not want to try out anybody’s Y product: it is designed for the cash poor, young who don’t mind body contortions for hours as long as they get to the other end in one piece. WT+/Premium Y is very definitely worth it because the seat is wider, the seat pitch (leg room) is longer and both the catering and in-flight entertainment is better. You have a far better chance of actually being able to get some shut-eye in WT+ than in Y at the back.
Beyond that, I have no idea whether/what frequent flyer status you might have. If you join BA Executive Club (it’s a nil cost option to join at the Blue (entry) tier), you will start to accrue tier points and Avios (Warning: this can become a serious game-playing addiction…!) §:-)
If you are able to spend a bit more, then there is also the option (there have been multiple postings before now on BT forum on the subject) of trying an ex-continental Europe dog-leg through LHR. This will give you rather more Avios, quite a few more Tier Points and all for many fewer pounds. Suggested itineraries include LHR-AMS-LHR (itinerary 1 on a separate ticket/booking reference for the flights at the beginning and end of your overall itinerary) matched to a separate itinerary 2 AMS-LHR-HKG-LHR-AMS. Itinerary 2 will be priced in €uros and you will be astonished how much cheaper this is than direct from LHR. This is because BA is seeking to attract hub traffic from the continent onto its long-haul services via LHR – and it prices accordingly. There are any number of alternative itineraries including places like Dublin, Oslo, Copenhagen (they vary in terms of cost and efficacy).
Almost every network carrier does the same and, for much the same reason, CX to/from HKG is usually a lot more expensive than travelling through HKG to somewhere else. Which is why I’m travelling to Myanmar via HKG on CX; what they were quoting for Business Class (J) was a deal.