Hi tiger_thomas,
I fly LHR-BKK about 3 times a year and find Thai econ pretty good, no seatback screens though. Seats O.K for long flight. No chance for upgrade , difficult even with elite status ( I'm ROP Gold ). Benefits of ROP depend if you are likely to be flying with them on a regular basis, if this is going to be a one off trip then what's the point. You may find it useful to join if you plan to fly on other Star Alliance partners, and for domestic flights in Thailand ROP membership will let you jump the line for standby/waitlist.