It was a 2011 European not solely British ruling that raised it from 50 to 70 yrs, composers already have copyright over their music until 70 years after their deaths, so The Beatles, who wrote their own songs, were already safe from copyright expiry. It was artists who made their names with songs written by others, such as Sir Cliff Richard, Shirley Bassey and Tom Jones, and Elvis and Sinatra who were threatened with a steady decline in their earnings. The law affects recordings made on or after Jan 1st 1963.
So actually major British pop stars of the 50’s like Cliff Richard have already and will continue to lose out on massive hits from the mid 50’s to 1963.
Frankly I detect you are trying to make this a British vs American issue, when it isn’t.
ANY works that have not already secured copyright 50 years after creation automatically BECOME public domain under the new Euro tradition.
This is why Bob Dylan, the Beatles, The Beach Boys, and others are having to open the vault if only for time-limited digital releases (and digital downloads have NO resale rights/potential).