Then there's bitstamp but I think they may only accept transfers in EUR and not GBP? Plus I am not sure they have much volume on their site if I wanted to sell?
Is that a problem when you are in SEPA territory? I believe exchange fees are very low to nonexisting.
So far Bitstamp seems to work, volumes are very far from MtGox but that does not matter much unless you are investing very large sums. It does seem to stop working when they are under heavy strain from fast price increases or drops but that's pretty much par for any exchange site sadly.
I think MtGox offers SEPA transfers too, but there is a long waiting list to get your account cleared.