To repeat what Antonio said, if you book a cheap flight with ryan air or easyjet book ahead to get a cheap ticket. If you wait until a few days before hand the ticket can be surprisingly expensive (you're competing with businessmen at this point). If stuck, there is a bus company - eurolines - that connects Barcelona and London, but the journey will probably be at least 24 hours. May be worth considering if your luggage is heavier than the 15kg currently allowed on budget flight operators...
_________________________
An English Bookseller in Madrid