No, definitely Priority Pass as it came from their official @prioritypasscom Twitter feed!
I now have a little more information. London Stansted (STN) is (so far) the only place that offers this facility. Here are the notes on the Stansted No 1 Lounge:
Cardholders wishing to use the lounge can also use the Fast Track channel through security by presenting their membership card. Complimentary Fast-Track facility only available to cardholders using the lounge on the same date. Follow signs for 'Premium Departures' after check-in, found on the left-hand side of the security lanes.
Is this the shape of things to come? If it is, then Priority Pass suddenly becomes better than before, especially for humble Y passengers.