Oyster and Contactless PAYG are now accepted for travel on Southeastern High Speed services between St Pancras and Stratford International stations. The fare charged is a special fare, like those for the Emirates cable car and the Thames Clipper riverboats and it does not count towards any daily caps. It will also split any longer journey (eg Romford to Baker Street) into three bits which will cost significantly more than the single journey avoiding High Speed 1.
However, for people travelling to Stratford (for the forthcoming Rugby world cup especially) it will offer speed and convenience. The adult single fares are £5.40 peak and £3.80 off-peak and all the usual discounted fares for railcards and kids exist as well. Travelcards* and Freedom Passes are not accepted on HS1 (*unless marked plus high speed).
This post somehow didn’t get published at the time but is dated correctly.