America doesn't turn it's nose up at the Q400. United Express have over 20 of them operated by Republic Airways, but unfortunately the aircraft don't have the range for the vast majority of 'regional' flying in the USA. The Q is great for sub 500 mile sectors, but at the regional I fly for our average sector length is 773 miles. The Q just isn't the right aircraft type to cover that sort of flying.
However, back at home in the UK, there really is very little need for the E175 on flights like MAN-EXT. The Q is perfect for the mission. The fact that the 175 has been brought in to fly GCI-LGW is just proof that the use of the E175 is more about misplaced pride and less about the lack of performance from the Q400.