I think it's not unique to BA's operations to BKK. If you look at TG's flights to Europe recently (such as TG910/911 London; TG930/931 Paris) almost all have been consistently late, especially the westbound sector. It's due to the re-routing to avoid Pakistan airspace. On my recent flight BKK-CDG, we left BKK almost on time but still arrived nearly an hour late.