Three really important issues are being intermixed here.
1) the laws of copyright ownership and protection vary from country to country. And the practicalities of copyright vary from country to country;
2) in the USA, the remedies available to someone whose copyright may have been infringed are dependent on registration. And as any lawyer will tell you, it is all about remedies, because nothing much else matters; and
3) in the UK, a new wrinkle has been added to copyright law with the ostensible purpose of making it possible to use material for which the copyright owner cannot be identified or located.
a) some may recall a relatively recent fuss where Scarlett Johansson suddenly had a bunch of partially naked photos of herself appear on the internet. As it turned out, the photos had been taken by Ms. Johansson herself, using her phone, and someone hacked into her phone and stole them. Ms. Johansson had a lot of success using the courts to prevent the re-use and publication of those photos because, as the photographer, she held the copyright for them, and thus was able to expeditiously obtain a number of injunctions that prevented future breach of that copyright; and
b) in Canada, unlike most of the world, our traditional rule has been that if you contract with a photographer to have photographs taken, the copyright on those photographs belongs to you, not the photographer. That rule has been recently changed by new legislation to match most of the rest of the world - the photographer is the copyright holder now. This change isn't as important as it may seem though - it has always been the case that the rule may be amended by contract. In a commercial setting, invariably ownership of copyright is dealt with by contract.