I think you must also include the general collapse of confidence in the British way of life with the end of the Empire, and the snobbishness/dislike directed at singing in any English local accent. It's not just class warfare, it's also parochialism. The Beatles managed it, and I dare say you can name others easily enough, but there's still the lingering attitude of "not really us". There's one (ar least one) member of my local club in Lancashire that finds Janet Russell's singing voice "too Scottish". In this context, the Mid-Atlantic/US accent is seen as acceptable because it comes from outside, without hangups.