Don't use Expedia unless you see a special offer. They are sooo expensive, I reckon about £200/seat more than elsewhere. No idea what going on with them, I used to use them often for flights and hotels but they have gone mad.
I did a search on Travelsupermarket.com but also try Kelkoo.co.uk and Kayak.co.uk, any could point to a cheaper deal.
For the Virgin flight they are only £617 on Virgins own site, how can Expedia get away with over £800? unless I am looking at something wrong.