This comes up all the time and I would just say that you should RELAX
Copyright in the UK is automatic - the lyrics are automatically protected by copyright
IF someone "stole" your lyrics then you would just need to prove you wrote them first - ironically posting them here would probably help with that proof!
The biggest question is: What are the chances of someone "stealing" your lyrics?
I haven't read your lyrics so please don't take this as a dig at your writing but I would say the chance is pretty much zero - and certainly less than the chances of you being run over - so should you stay in your house all the time
IF you wrote the perfect set of lyrics a publisher/artist would want YOU - not the lyrics
The example I like to use is that they would want "the goose that lays the golden egg, not the golden egg"
The other benefits of posting lyrics is the feedback you get, which should help you improve
You might also find a collaborator who could bring your lyrics to life in a song (with your permission)
For me the benefits of posting songs/lyrics FAR outweigh any minute risk the work might get "stolen"
There are copyright services available (I would would recommend the Guild of International Songwriters and Composers over the internet based services for lots of reasons) that can formally protect your work if you are still worried
I would therefore relax and concentrate on improving your writing, and a key part of that is getting impartial feedback on your work IMHO, so I would continue to post lyrics/songs