When booking it asks flying out time. So I enter 1pm, but when it gets to payment summary, it says Security Fast Track at 1pm, but I'll be using it a 11am. Should I put 11am to begin with? It's not very clear.
It all depends. On quite a few occasions, I've been stuck in a queue and had to run for the flight. I don't want the stress of that again, it ages me ;)
I have just done this to see what happens. The times are listed I just touched 11am and this time appeared on the confirmation slip at the top of the payment page. I did not have to type in a time just select from the list. Why not try again
The last few times I've flown from Stansted, I've got through the ordinary security desks quicker than people who were using the fast track ones.
As long as you've entered the correct departure time during the booking process though, your booking should still be valid. (i.e. if you put 1300 the airport has no way of knowing whether you'll turn up at 0700 or at 1200; you should still be allowed to use the fast track system on arrival anyway.
PS: Having been stuck in some long queues at passport control upon returning to Stansted (and then sometimes finding that the automated system doesn't recognise me, forcing me to queue again for a manual check), I think that I'll definitely be making use of the accelerated passport control option for my return journeys in future.