One thing I haven't yet addressed in this scamper through the 1950s UK charts is just why that general audience split into two irreconcilable halves mid-decade - in America as well as Britain.
I had assumed it was essentially economic, ie the targetting of a new group of people with disposable income, as typically summed up below:
In 1955 teenagers had economic power, often accompanied by a consumer frenzy to equal that of their parents. They created a new market, which from that time on was flooded with products made especially for their consumption [...] And, of course, records. The 45rpm single had just made its appearance, and the portable record player allowed teenagers to take over the world of sound. Until then, to play music they had been dependent on the good mood of their fathers, who were usually in charge of the cumbersome record player enthroned in the front room.
But an article by Jon Savage in today's Guardian traces the origins of the divide back to the first atomic bomb - and offers a related reason for the scale of the Beatles' success in Britain:

