There is also the consideration, that if you want to be able to advertise "Fly for £1", then it must be possible for people to actually pay £1 on one or more dates on the route specified.
As to whether this market strategy has a net profit (i.e. revenue gained because people associate Ryanair with flying for peanuts, both direct and ancillary, is worth more than the cost of subsidising some seats) is worth deliberating as well.