Porpoise.
Why do folk shoot video in that orientation?
From the shape of the dorsal fin I would say that bottlenose dolphin is more likely than porpoise.
They appear to be ignoring the yacht, which is behaviour more like porpoises than dolphins. Not Harbour porpoises, though - too big. Pilot Whales look likely; see http://en.wikipedia.org/wiki/Long-finned_pilot_whale
Gosh, I was expecting a simple answer. They were just off the Cumbria coast. They came to visit 3 times over two hours and did play around under the boat.