The same issue arises on BR67/68 TPE-BKK-LHR.

I regularly buy bottles LHR. You must ask them to put the bottles in a special sealed bag with the receipt. Then at BKK you just put the bag in a separate tray and they have no problem.

If you buy duty free and do not have the shop put it in the special bag, then they WILL confiscate it at BKK. You must go through security before reboarding.

I never buy anything at TPE, so I don't know if they have the special bags there. At LHR duty free they will look at your BP and ask if TPE is your final destination. If you say it is, they don't give you the special bag, assuming the flight is non-stop. Many Brits fall into this trap, losing their stuff at BKK. You have to tell them that you will be transitting and need the special bag.

Though not really allowed, I have seen pax ask to leave stuff on the plane. I'm guessing if you are diamond/business class, they might do it as a favour. Bear in mind, though, that there is a crew change and the entire plane is cleaned.

This is such a common problem that at the gate at BKK they sometimes have a sign to warn people. But it's all rather late then!