This was a topic I was going to cover at the 2005 Con.
The short answer is that within certain limits you can register pretty much any name as a new company. Some limits are obvious (you cannot register the name of an existing company) whereas others are a bit more obscure (certain words are prohibited such as "Royal" or "Imperial" as they imply a connection with the Crown). Names which are discriminatory or offend against public decency are also likely to be rejected.
You can search Companies House free of charge to see if your name is in theory available. The website is www.companieshouse.gov.uk I think. Look for the free company information section.
Obviously if you register a company name which is too similar to an existing company you may find that you will then be sued for, inter alia, passing off by the existing company. You may then have to surrender that name. For example, you would be unlikely to avoid threats and litigation if you tried to register "Booots".
Philip
What do you mean, bears aren't supposed to wear hats and a tie? P3.2ghz / 1 gig / GeForce FX 5900 128meg / WinXP home