I think the law on copyright may be a bit different in the USA and the UK.

As I understand, in the UK, if you write something, you've got the copyright, regardless of any process of registering it.

But if money comes into the picture, at some time you are likely to have to prove that you wrote it, and that therefore you own the copyright, and there are various ways of doing this.

The simplest way is to send it to someone (such as yourself) in a properly sealed envelope by recorded delivery; and then put it somewhere safe, unopened until at some time you might have to prove you wrote it.