101, when you list you are not buying a new ticket. You are just adding your name to the list of staff trying to get on the flight. You are only charged when you buy the Cat A ticket.
For a place like LHR list for the 1st flight you can make. If you don't get on the staff will list you for the next one as needed. if you are worried you won't get out of LHR you can also list for flights out of other UK ports for the same day if you could get to them.
The don