I'm just going to toss in one other difference betwen the U.S. and the U.K. Possibly overlooked since this is a pro pilot forum.
The flight levels (Class A airspace) REQUIRE an IFR clearance (except for rare SVFR). In the U.S. there are still a fair number of well-off private pilots who like to hop in their turbocharged whatsis, strap on the nasal oxygen, and fly VFR in the 'teens - without having to get involved with ATC over the wide open spaces (except towers, or the high-volume big-city airports with Class B protection.)
The average procotologist in a P210N flying from the L.A. basin to Aspen or Lake Tahoe would consider it "unwarranted government interference" to have to file IFR to cruise at 17,500' on a clear day (let alone 7,000' or lower). And they have a lobbying group - the Aircraft Owners & Pilots Association (AOPA).