It should refer to your organisation and business and be easy to remember when seen in an advert or article. It should also contain recognisable words if a good response from search engines is hoped for.
Many people want a .com address but if you do not trade outside the UK then you are more likely to be found by someone who clicks on "UK Sites Only" on a search page, if you have a .uk address.
New Top Level Domains (TLDs) have been created, with .info, .biz and .name for instance. These can be registered, if available, and can help to make your domain name more memorable but people who are not sure will try .co.uk and .com first and may not even think of .biz etc.
We can register your domain(s) for you and, if we are building the web site, will submit it to the major search engines as a matter of course.