BRITISH SEA POWER Valhalla Dancehall [Rough Trade] British Sea Power is known for big ideas and an even bigger sound. On their first three albums, these wily Brits transformed themselves from artsy thrashers into U2-grade stadium-rock flag wavers. That the group was nowhere near popular or accessible enough to play stadiums made the whole thing all the more confusing—and intriguing. Then came its fourth album, Man of Aran, a largely instrumental...