I've been "contracting" or "Freelancing" for 15 years.
If you want to go contracting you will need to set up a company or set up to work under an umbrella company.
Doing pure freelance work is very hard to do which is why most of use do contracting with some freelancing on the side.
As to rate it depends what the industry is what experience you have and what the language is.
Have a look at
http://www.jobserve.co.uk to get an idea of what is out there and at what rate for contract work.
Join The Federation of Small Businesses
http://www.fsb.org.uk/ get your self some indemnity insurance and an accountant to keep you on the straight and narrow.

( how far you can bend the rules by before HMRC get pi55ed off at you )
Hope that helps.