Vicki is right - copyright is automatic
The only issues arise is if someone rips you off and copies your song and earns a load of money you would need to be able to prove that you wrote a song when you did
The first thing to say is that the odds of this happening are infinitesimally small - there is probably more chance of getting hit by a bus
However, if you did write a hit and send it to a publisher/ label they would not want to "rip you off" - hits are so rare that they want the goose that laid the golden egg so would not bother trying to steal the golden egg!! - they would want you to write more hits!!
There is a thing called the "poor man's copyright" where you post a copy of your song to yourself recorded delivery and sign across the seal with your song in it - however, I can think of a number of ways to fraud this system
There are service out there that claim to give formal protection but very few have been tested in court as it is so rare for this type of dispute to happen - but they prey on fear to make their money
If £60 a year is too much for you to get unlimited copyright protection, expert legal advice on music industry contracts, general industry advice, song reviews and the other benefits of membership to GISC I think it is safe to say that you would not have the money or resources to defend a copyright claim if a song of yours was copied. For someone in your position who is concerned about copyright and needs someone to check music industry contracts the £60 GISC membership is a no brainer IMHO
Even with formal copyright protection there is likely to be a court case - are you willing to spend thousands of pounds to fight a label knowing that you "should" win but there is always a risk when going to court
I would personally say that the chances of this happening are so small you do not need formal copyright protection. Posting lyrics on this forum, putting the song on YouTube, SoundCloud etc. all help to provide proof of when you wrote a song
Relax and carry on writing more songs