Now for the snag ............. the Embassy said that we have to check with the airline to be sure they will accept travel on a CoI. We are travelling this time with Jet for the extra baggage allowance. I've phoned Jet to ask the question, but rather than give me a yes/no answer, they insisted I submit my question via e-mail

Assuming the worse, the key part of the CoI is to enter Thailand using it. As a workaround, he could use his British passport at check-in, and not produce the CoI until he arrives in Thailand. OK, plan B works until we remember that he is travelling on a one-way ticket, and no visa. Officially, the airline can refuse him without a valid visa.
It's been mentioned before that an easy workaround for snag 2 is to book him a 'cheap' ticket out of Thailand to a neighbouring country. Now my question, if we need to invoke plan B, where is the cheapest destination to buy a one-way ticket out of Thailand to please?