In the reverse direction you would probably be in with a good chance, unofficially. In London you probably would not be allowed to do it unless there was something else forcing the change; and if you have checked luggage I doubt it would be wise to do so either, unless you had a good 2 hours to spare, and they would be reluctant to move you. The place to enquire is at the Special Services desk in front of the Concierge desk of CCR. It's not always staffed, but someone would be along at some point, or you could ask the lounge agent by the secret door.