With latest austerity cuts there is talk of adapting something of the American IRS tax program for ex pats with a British passport but that is at least a year away if it happens at all.
As a disinterested observer it would make sense for H.M govt to look into that program with so many citizens living abroad it would be a huge revenue stream.
The US govt institutes it and only 15% of Americans have passports and much less live abroad.