Well, I've now returned from my first class round trip so here are a few comments...
In the end I decided to book my flights on Thai as I thought this gave the best combination of destination (who doesn't like Bangkok?), reputation and price, and on an A380.
I booked the flights on Opodo after searching on Skyscanner. Opodo weren't quite the cheapest but I thought it made sense to book with a company I'd heard of. Big mistake. The actual online booking process was seamless. ie they took my money promptly and confirmed the flights within minutes.
No problem so far. The problems arose when Thai altered the schedules and put a B777 (C & Y only, no F) on the midday flight (which I had booked), with an A380 now operating the evening service (normally a B777). Opodo informed me of this by sending me an email copy of the Amadeus booking but without any explanation or mention of any fare adjustment because of C instead of F or offer of a change of flight. This meant phoning Opodo to seek clarification. Initial contact was promising-a well-modulated female voice assured me that my call was important to them and press 1 for this, 2 for that etc. At this point things started to go pear-shaped. The agent I finally got through to was in Hyderabad or somewhere similar and seemed barely to understand English and certainly couldn't grasp what my call was about. When he finally got the gist of the problem and put me through to the right department their system crashed and I was told to call back. Several attempts later I finally got through to someone who understood the problem but whose advice was to 'call Thai, they will help'! This I thought would lead to a roundabout of calls with neither taking responsibility. Thankfully, I was mistaken and I ended up speaking to a most helpful agent at Thai who changed my booking to the later 380 flight, offered me a choice of meals and confirmed my preferred seat (2K as recommended by SeatGuru). As an aside, Opodo stiffed me for two fees to select seats on both legs. Not much in the scheme of things but Thai would have allocated the seats without further charge, so won't be using Opodo again.
Thai flights depart from a shopping centre near Hounslow aka Terminal 2. My recollection of T2 is of the old Queens Building but this new T2 is nothing like it. For me an architectural nightmare which you enter via a series of crowded elevators and consisting of two separate buildings. The TG flights, needless to say, departing from the furthest building.
Thai is apparently part of Star Alliance and the check-in and passenger handling is dealt with by Star Alliance-uniformed personnel. My check-in was with a morose Eastern European woman who seemed determined to ensure that my first class experience didn't start with her. In fact, the whole Heathrow experience was rather disappointing; a fact that I put down to poor staffing levels as much as anything. For instance the buggy which should have conveyed me to the departure lounge (I was using the SQ lounge) was nowhere to be seen. The SQ buggy with driver was there but 'sorry not Singapore, you Thai' meant I had to walk. It is a LONG way! There are travelators but there is also a sign saying something like '17 minutes to departure gates'. A further observation - I didn't speak to a native English speaker (other than a fellow passenger) from arriving at T2 to boarding the aircraft.
The SQ first class lounge was excellent with a generous menu of hot food available (didn't partake) plus the usual snacks and choice of drinks but it wasn't until boarding the A380 that the first class experience really started. Greeted by name from the outset, nothing was too much trouble for the two dedicated F class cabin attendants. The menu was great (I really enjoyed the caviar), vast choice of movies on the entertainment system and a comfortable, secluded seat with all amenities close to hand so no need to talk to your fellow pax unless you (and they) want to. (There are a number of videos on Youtube showing the accommodation and menus etc). There are twelve F seats on a Thai A380 and they were all occupied on both legs of my trip. Once fed, watered and entertained, the cabin staff offer a pack of 'leisure wear' (pyjamas) and make up the seat into a bed whilst you change in the extremely spacious forward loo. If I'm honest, I found the bed rather hard and 'under-pillowed' but it is a genuine 'lie-flat' and full length. I'm 6' 2" and fitted in with room to spare. I did manage to sleep and the 11 hour flight passed very quickly.
The first class experience continued on the ground at Suvarnabhumi with ground staff meeting disembarking F passengers and conveying them on buggies to the fast-track immigration counters (and it's a long way!). Once through immigration more ground staff are waiting to accompany you to the baggage claim and push the trolley with one's bags through customs to the waiting driver. (The driver was from my local contacts, not provided by Thai). OAT on arrival in BKK was 36 degrees C!
The return trip (a midday departure) was pretty much the same but in reverse; dedicated F check-in where one sits in an armchair with a drink whilst a ground staff member checks in for you, buggy to the Thai F lounge (splendid), superb personal service on board, another set of pyjamas and another snooze meant the 12 hour flight passed quickly. Once at Heathrow one is dealt a dose of depressing British reality. Sure, there was a buggy meeting F passengers but only one (a five seater) so this meant a wait while this one buggy took its first load to immigration and then returned to pick up those of us who didn't fancy the very long walk to arrivals. Thai's responsibility I know, but typically British poor service.
We were all given cards directing us to the fast-track immigration; unfortunately so were the F and C pax from the Eva Air flight (also from BKK), the ET flight from ADD, an AI flight from the sub-continent and several others) so there was a very long queue. Bags were already waiting on arrival at the carousel so through customs and into the British evening with a temperature of 6 degrees.
All-in-all an enjoyable experience; would I do it again? Yes, definitely if I had the spare cash available
ExXB thought it would be very expensive:-
You realise that you are looking at £7,000 each way.
In fact it cost me less than half that for the round trip.