You can argue this is like a post-contract condition, like Thornton v Shoe Lane Parking (such conditions won't be effective for the first-time contract but will be for contracts thereafter)
I would imagine it will be construed as a stricter of term ie LHR 30 mins if they have it, 20 mins for other airports.
Hong Thai isn't a a contract term (since you don't decide the itinerary anyway), just an annoying practice. Something I get around by doing my own holidays.