For comparison, I was gifted 10,000 Avios 3 years ago when on a BA flight (not saying your flight was BA, but they are the only one who uses Avios on the direct BKK-LHR route) from IST-LHR the curtain separating Business from Economy (was a Euro config 767) blocked my view of the TV and the headphone socket was faulty too. I mentioned in my complaint email that I was unable to see the safety demo video (which I wasn't) and I suppose they thought I may take it further (I wouldn't have) so within a few weeks I had the avios in my account, which was quite a lot compared to what I also got from the flight itself.
A friend of mine was on one of their early A380 LAX-LHR flights which suffered from an IFE system failure and the duration of the flight had no IFE at all. She was apparently given 20,000 avios without even needing to complain. I am not sure which cabin she was in, but I do know she flies that route every 6 weeks or so, so she may be at least bronze level. Then again perhaps the compensation was upped as it was quite embarrassing for their brand new plane to suffer such a thing.
I don't think there seems to be an hard and fast rule, or if there is then no one has applied it in your case as 5,000 avios for such a thing doesn't seem all that much, but then again, you did fly and did sit in the seat for that time so they are maybe less willing to gift anything more as they got you to LHR as intended and you agreed to sit there.
Having said that, if the seat was known to be broken before boarding started then they would have had to sort you out somehow or get an engineer, but clearly this has happened on the inbound flight, possibly the pap before you did something and didn't bother to mention it. There is only so much you can expect the crew to do. If the inbound crew were not aware of it and the seat wasn't damaged enough for the cleaning team in BKK to notice or the new crew to notice on a pre-flight walk around then It is hard to imagine just how broken it was, compared to just being uncomfortable due to wear and tear. The TV Screen not working is bizarre though and would suggest the seat infront also has a problem!