Ian, I may be labouring a point here but it seems to me that two separate issues have been conflated here. Firstly, there is the issue of interlining between carriers, whether within or external to an alliance. I think you will find that BA do indeed interline in most instances where there is a single ticket – travelling from Glasgow, this is a frequent experience for me and I have availed of this service both within oneworld (from BA to QF, CX) as well as beyond oneworld to the likes of Air Malta and EVA.
The second issue is that of separate tickets and I have less sympathy here. You COULD have purchased a combined Thai – BA ticket for your routing and paid considerably more, no doubt, for the privilege. I suspect that would have ensured through checking. You chose to cut your costs by buying two tickets and, IMHO, it is reasonable that you loose some of the benefits as a consequence. The fact that HAS chose to over-ride their obligations in HKIA is neither here nor there. Had the Thai flight been delayed and you missed your BA connection in BKK, they would have been imposing unnecessary costs on their principal and may not have been very popular as a result. Finally, to say that BA lied to you is, perhaps, a bit strong – give their folk the benefit of the doubt here in trying to soften the reality – that you actually did not have a leg to stand on with your two tickets!!