It sounds to me like the question is whether it is a problem transitting with alcohol or not...not one of entering Qatar with alcohol.

Even if you are flying premium you will still clear security again after you deplane in Doha and enter the terminal before you are allowed into the Duty Free or the lounge area. This is what I suggest: Buy your stuff in LGW. It is only 15 pounds, so is it really a tragedy if you loose it and with any luck you might not. If you are traveling with someone else, split up the booze. The security staff at Doha aren't the greatest and they may just miss it. If they find it and take it, you can always buy something else in the duty free there or wait until you get to BKK.

The last 2 times I entered Doha itself I had booze with me. Both times I got it through with no problem. I tend to buy the smaller pint size, so maybe they are just not in tune with this size.

Don't expect too much from QR in J class from Doha to BKK. They are using a crappy plane on this route with crappy seats. The flight this plane is on is the overnight from Doha and its the A340.