You can always route back direct to LHR from BKK.

Just choose the connection to AMS the following morning from LGW. The taxes from Gatwick will save a tiny bit too.

I’ve often done this itinary. On the way out I also try and earn extra TP by travelling via HKG or SIN and then keep my eye open for a F award seat. I’ve had success also routing KUL but you won’t earn TP on the KUL BKK leg. At least I didn’t

AMS LHR HGK BKK, BKK LHR (finish up here) LGW AMS should be attainable for the figures you mention.

Wil net 360 TP. If you go via SIN it’s an extra 20 TP (as is KUL but you’d then lose 40 for the KUL SIN leg so is actually 20 worse)