Qatar IS Very Good but as far as getting from London to Bangkok goes, Cathay Pacific is ALSO a superb airline and even in Economy you'll be given First Class-treatment! :-)
I found the Royal Thai Embassy in Phnom Penh to be very friendly, quick and enjoyable for getting a Tourist Visa--you might want to consider going there. Plus, PP is a wonderful city to spend a couple days in:-)
I'd recommend going to the Royal Thai Embassy in Phnom Penh as I found the staff there to be very friendly and outgoing regarding getting the kind of visa you want and PP IS a decent city to hang out in as you await the visa service.