I got a ticket price for TGV Barcelona to St Pancras £145 and Monarch from Barcelona to Gatwick £61 including all fees and taxes and one hold bag. Add train fare Gatwick to London £18 and you have £78. That's less than half the TGV fare and a bit quicker (roughly six hours instead of nine by TGV). You pays your money . . .
(However, I should own up and say that I'm with OFSO on this one and as soon as the High Speed Line is extended from Barcelona to Valencia, I'll be on it!)