This might be the answer your looking for…

From businesstraveller.com:

Qantas and BA announce JSA revamp

Qantas has announced a restructuring of its Joint Services Agreement with BA, as part of a major new “International Strategy” for the airline.

The carrier says the changes will “consolidate Singapore’s position as the primary hub for the JSA relationship”, with BA swapping its B777 aircraft serving the London-Singapore-Sydney service for a larger B747 jumbo.

Qantas will continue to serve routes from both Melbourne and Sydney to London (via Singapore) with its A380 superjumbo, and also plans to open a new lounge at Changi.

The restructuring will also see the two carriers concentrate on operating JSA services from their respective hubs – BA will stop operating flight sectors between Bangkok and Sydney from early 2012, while Qantas will no longer operate sectors between London and Bangkok/Hong Kong. The move will also see BA increase the frequency of its London-Hong Kong service to 17 per week (from 14).

Source: http://www.businesstraveller.com/news/qantas-and-ba-announce-jsa-revamp

I am pretty sure that BA would not be dropping the LHR-BKK route as that would only leave Thai Airways International as the sole operator to/from LHR, and there is no way that they [BA] would allow another airline [TG, in this case!] to have a monopoly.

I think you’ll be fine, but I would always advise you to contact the airline directly if you do have any major concerns.