Hi Keiron,

I don't think you can fly direct London- Thai resorts because those regional airports are small and can't accommodate the big planes that are used on long-haul international routes. There's also the issue of certain airports being the exclusive domain of certain carriers.

So, a few options spring to mind...
1. fly London-BKK and get a domestic flight to Phuket or, on a smaller scale, Koh Chang. These places would probably meet your requirements of snorkelling, beach, restaurants and some 'high-end' resort hotels.

2. fly London-Bkk and hire a minivan, limo or 4 wheel drive vehicle and travel to a place accessible by road - Hua Hin or Jomtien - I don't think they are renowned for snorkelling but have some nice hotels (quite a few absolute beachfront) many restaurants and plenty of shopping.
You might like one of the nice 2 bedroom suites at Cabbages and Condoms just out of Pattaya (absolute beachfront resort with lovely gardens) or a 2 bedroom villa at Rabbit Resort - across a paved, narrow roadway (no cars during the day) to the beach.

I've just come back from HH and there are certainly some upmarket hotels there too - many absolute beachfront.
Do a google search on hotels in these areas and you'll get a lot of information.
3. You could also check the luxury resorts on Koh Samet - but they could be a bit boring for your teenagers (most of the action is in the main part of Koh Samet and if you like luxury hotels I think you'd hate the main part of KS - personally I think it's grotty and very over-rated, except for the couple of expensive options)