British copyright like the copyright of most countries exists the moment it's created because you do not buy copyright like you do in the USA.
Works the same way in the US as far as creating the copyright goes. However once it exists It can be bought and sold.
If a US company buys the full rights and then purchases a US copyright
It doesn't work that way in the US, nor did I in any way suggest that it does.
The US used to (pre 1976) require registration for a copyright to exist at all, and the clock on expiration started at registration. Post 1976 the existence of a copyright is automatic like it is everywhere else, but registration didn't go away. There are a few extras that a registered work gets (statutory damages mostly.
A foreign authored work could and still can be registered in the US, gaining a US copyright. But, post 1976, that doesn't start the copyright term over like it did pre 1976.
US laws that does not affect the material falling into the public domain outside of the USA and there's nothing the US copyright owner can do about when it happens outside of the USA.
True, but they can prevent someone else republishing inside the US until the US copyright expires. That's all I really ever claimed.