I had the same problem with air India 2 months ago. I flew from Heathrow to Delhi they were unable to issue boarding pass for the Bangkok leg because I did not have a visa but I was able to resolve in Delhi as they advised that was the only way. I had a return flight booked within 90 days. Just as an aside I was also quizzed on arrival in Suvarnabhumi about my reasons for visit as I had only been in Uk for 3 weeks and had visited Thailand four times in the last year advised to get a visa next time
They will email me with the consultation date but the consultation will be by phone. I do not know yet how critical it will be, but I need to take the call and will fly back if needed