There's no such department.
The author of any work automatically owns the copyright to that work (unless that work is written in the course of his or her employmment for someone else, who would then own the copyright).
There are commercial companies who operate unofficial copyright registers. However, there's no real point in using them because, as stated, you already own the copyright to any novel which you've written. If you really wanted to ensure that nobody could steal your copyright, you could simply ask a solicitor to keep a copy of it in his safe, together with a note of the date that you handed it to him.
However, I've never heard of any author actually using either a commercial register or the services of a solicitor. It's simply not necessary.
http://www.ipo.gov.uk/copy.htm
Chris