is automatically protected by copyright from the moment of its creation. You don't need to do anything - certainly there's no need for any kind of registration - other than be able to prove that you are the creator of the work, and can prove the date of original creation.
If you want a good (but not foolproof) way of protecting valuable text you should produce a copy of it both on disk and on paper. Put the copies in an envelope and send them to yourself by special delivery. Before posting, seal the envelope in such a way that it will be obvious if it's opened. The envelope will arrive with a nice shiny label showing the date of posting. Don't open it - keep it ready for the day when you may need to prove your right to copyright in the material. If you are exceptionally nervy you can send a second copy to your solicitor for safe-keeping.
The disk in the envelope will of course have a time/date stamped copy of the file, which is an additional check on the date. This method isn't infallible, because it would theoretically be possible to open the envelope and replace/alter the contents.
Alternatively you can pay £60 to the UK copyright service, and they'll verify your copyright for a period of 10 years. As I said earlier, this form of registration is not necessary - your copyright still exists if you don't do it - but it certainly helps in the event of a court action.