You will need to search each airline ffp to see if they allow you to join and then retro claim,.Some ffp's only allow to credit flights taken after signing up. Some may allow you to sign up and then claim for flights taken up to X days before.

If you had your Air India freq flyer number in the reservation other airline ffps may consider "already credited to another ffp", even though earned nil ff miles/points/avios. In that case would need to get TG to uncredit (may~may not be possible).

Someone from GB joining Air India ffp is a little unusual for Thai flights, unless you plan a lot of Air India flights. Seem to be few threads on Air India FFP. Most people would join an EU Star Alliance ffp or UA (miles never expire).

Link--->Which Frequent Flyer Program to Join? Help Is Here!