For a UK based company that will never sell anything or provide any services outside of the UK it may be sufficient to just use .co.uk at the end of the domain name.

If you really want to stop other companies from using your domain name but with a different extension, then it might be worthwhile to register the .com extension as well. Many larger companies also buy the .org and .net extensions to cover all angles. Note that new extensions are suggested that will represent certain types of businesses so there is reason to keep an eye open for when that is happening.