Dick is more or less correct, except techinically it is the registration of the copyright that is an unsubstantiated claim, not the copyright itself. When people speak of "copyrighting" something, they really mean registering it with the copyright office. In theory, however, copyright exists when a work of art is created, and it belongs to the creator unless he or she assigns it elsewhere. This is true whether he or she chooses to register it or not.

If the creator does not register the copyright, someone else may try to do so. But this does not mean the person who registered it owns the copyright, only that he has claimed it. If this registration is tested in court, and the person who created it can prove that he did so, the registration may be declared invalid. In this sense, the registration is just as Dick said, an unsubstantiated claim. Most such registrations are true claims...but in the realm of traditional song, many such registrations do not stand up in court.

This is how it works in the US, in any case. The UK may be different.