Hi BL
many opinions on this one. I run a partnership and a Ltd company, one VAT registered the other is dormant.
If you are a limited liability company and registered with companies house
www.companieshouse.gov.uk/ then being a ltd company means that you have already registered (with companies House).
You do not have to register with companies house if you go over the VAT threashold, just with Customs & Excise.
If you are running as a sole trader or partnership etc then there is no where for you to be registered other than customs & excise for VAT and inland Revenue for paye, ni, self employment etc which your accountant should be able to help with if needed.
I would also agree with some of the other comments, although you can pick a name that someone else is using if you are a sole trader or partnership (other than a trademark) its generally best to pick a name that is unique or makes it clear what your business is. If your company name is bizzie Lizzie then this suggests that your name is Liz and that you are very busy!!!!
Hope this helps
Chris