Ryanair's funny exchange rates
 

No, not the awful rates offered if you are buying a GBP-priced flight with a Euro debit card (or vice versa). Rather, the prices offered for the same thing, in either currency.
For example I am currently looking at mid to late August flights STN-BCN. Outbound is £68.34 per person + £141.96 for the "family plus" offering - there's three of us. Add a return and it's £122.03pp + £139.96. But book that return flight separately it's €138.03pp + €139.96! So the "family plus" component is the same in pounds and euros! Even if your British bank has a fairly heavy foreign currency loading you are still better off buying the return separately. The only drawback I can see is if your outbound gets cancelled, you will only get the refund for that one.