If you are booking a Ryanair flight, especially if it is quite expensive, you can save money by doing this.
Book you return flight separately, and outgoing flight separately. Because when you book a one way flight (return flight) it will be charged in Euros and not pounds.
So say a £50 return flight will now be 50 Euro, you save because of the exchange rate!
I can't be sure if it works on all flights, but it has on mine so far!!!