Yep. For a couple people, the bus is cheaper, and glad to hear the BA discount still works. I think it worked out to be 14 pounds pp.
Can a cab handle 7 people? It'll cost somewhere south of 100 quid, depending on your negotioting skills, but that's not that bad on a per person basis.
Check into the folks from stut's post -- might get a better price, but at least you know your top end is about 100 pounds.