If getting yourself registered as a brand new company is so easy, what are the advantages of buying an empty limited company and changing its name? There must be some, or people wouldn't do it.
1. Having a company that appears to have been around for longer than it actually has (although it's pretty easy to find out it's history - e.g. when directors changed).
2. Getting the company name that you want. There are a number of people that form companies and register them as dormant just to squat on the name - just like people do with domain names.
Edit: forgot to add:
I'd recommend registering a fresh company. It's quick and easy. Buying a company, changing directors, making sure there's no awful legal history with it etc probably isn't worth the extra cost or effort. It's nice to have a clean slate to start with.