That is correct. Once the Javelin services start running later this month, Games Travelcards will be valid on them, but other Travelcards and Oyster Pay As You Go (PAYG) will not.A bit confused! Ordinarily one cannot use an Oyster card on the Javelin services but can use the special Games Travelcard between St. Pancras and Stratford International stations?
Games Family and Games Volunteers can use their special pink Oystercards to travel on our High speed services between St Pancras and Stratford International, both in the run up to the Olympic Games and in the period between the Olympic and Paralympic Games. This is only valid if the Accreditation card is presented with the Games Travelcard.
Please note no other Oystercards are accepted on High Speed services.
No, it won't be valid on HS1.
From East Croydon, I would consider changing at London Bridge for the Jubilee Line, which will probably take a similar length of time to changing at St Pancras, but without giving SET any undeserved extra money!