Hi jacksun.
Boydie already gave great advice but I'll re-iterate it:
a) No-one is going to steal your song.
b) There is no process for copyrighting a song in the uk
If you're song is truly excellent, get it on youtube. That's really the best way to protect yourself as it's dated, it's public, and it's not going anywhere anytime soon.
Here's the flip side... if you want to get this song published then don't put it on youtube, as no artist will cut your track if it's already been in the public domain.
I would advice creating a separate youtube account just for this purpose and posting the song there privately. That way no-one will have seen (heard) it, but it is there should you ever need to back and prove anything.