Curious why anyone would take PE (especially if paying 300 for an upgrade one way) for this price when you can get LHR-BKK on Gulf (J) for just over £1k with all segments on a Dreamliner with Apex Suites
I suspect for the following reasons:
1) Because it saves £450 (A £1800 does a lot for a family of 4)
2) It earns miles in decent programs that will help people maintain or get status.
3) They have good transit times (unlike your GF suggestion which requires an overnight not to mention extra ££ on a hotel)
4) This goes to multiple destinations and not just BKK
5) Likely better cover in the case of IRROPS due to multiple partners.
Having said that I do see your point. Personally if I was prepared to bid for an upgrade each way and wanted to fly J I would take the GF option but can see why a lot of people wouldn’t.