The Geo Metro WAS a 45mpg car, and it topped out around 80-90mph on the flat. The thing is that it doesn't pass modern safety standards.
Safety standards should scale with vehicle size, from few on lightweight cars, to many on heavy cars. This would effectively 'tax' heavy, less fuel efficient cars, while leaving economical cars economical. That might sound mean to the purchasers of eco cars, but frankly, the alternative is motorcycles if you want to go 100mpg.
If a manufacturer wants to have 'best safety in it's class, that's up to them, but we shouldn't put safety limitations on vehicles that destroy their cost and mileage when we're trying to get efficient cars out there.
Note that if there is a change in exchange rates that makes the British engines affordable in the US, Ford is certainly willing to sell the cars here. Just not if they're losing money or not making sales due to the price.