Can a foreigner in Thailand use both old and new passports for immigration purposes?

@Brandon ***********

That is true. It isn't an Embassy service. It is outsourced by the embassy. Got one maybe 3 weeks ago. The only question is whether VFS will play ball if you haven't completed the whole process here. You can but ask - and they seem quite a decent, helpful bunch. It's all in the words. It could also be that if you popped into the British Embassy they would say a simple "No" - they are not very helpful people - rather than advise asking VFS Global. I note that the letter is very new, so I suppose another question is whether they've now stopped supplying the letters to VFS Global. All this said you never did need that letter anyway. So the final question becomes whether the individual immigration officer is fully appraised.