Ryanair currently have £2.50 flights no taxes on in Fri 22nd May 09 - Mon 13th Jul 09 booking until 10th May:http://www.ryanair.com/site/EN/
If you sign up to Entropay then this gives you an instant virtual visa card which you can top up immediately (minimum £10) online. Ryanairs website recognises this card as Visa Electron which means no card admin fee so you really can get flights for £5 return!:http://www.entropay.com/
Use this website to find the cheapest day to travel. The flight offer only started this morning so Skyscanner prices are not yet 100% up to date but it gives you a rough idea of when the cheapest flight is so you can then go to the Ryanair website:http://www.skyscanner.net/month
I have used this method to book a flight and it works.
Reps to pototea for mentioning entropay...Enjoy :D