In the UK copyright is automatically established as soon as you create something
There is no formal method to copyright a song in the UK
You would simply need to prove that you wrote a song (lyrics and/or melody) when you claim you did
There are services where you can pay for a copyright (but what happens if/when they disappear?), the Guild Of International Songwriters and Composers offer a good copyright service and there is also the "poor man's copyright" of sending a copy of the recording to yourself and not opening the envelope
I do think uploading to websites would help with proof
However, it is worth questioning whether you actually NEED to copyright a song
How likely is it that someone will rip it off and make lots of money?