There are a few issues with contracting in the UK. The UK govt. have a rule called IR35.
It would all depend on your contract if you fall inside IR35 or not. If you do , you will pay tax on most of your pay. If you do not then you can set up a limited company, pay yourself the minimum wage (£10k approx) and take the rest in dividends which is taxed at a lower rate (about 20%). With a Ltd company and outside IR35 your take home pay will be about 80% of gross.
With Ltd company and inside IR35, will vary so can't really say but would probably just be normal tax.
Also, you can use the services of an online account, I use SJDAccountancy (cost me £80 per month, but that includes your end of year account, payroll etc.)
Due to recent tax changes in the UK, the limited Company is probably the best option.
Another option is the Umbrella company. There are various suppliers out there. They basically act as your employer and you can claim back a lot of expenses. Beware though as the govt. will ask for receipts if they perform an audit on you. The Umbrella organization will raise invoices for you and also do your paye etc and then give you a wage.
Using the Umbrella company you can take home between 80 - 85% of gross but is risky if you try to claim expenses that you do not actually incurr.
You can get good advice on contracting in the uk on
http://www.contractoruk.com/
The forums are good for Q&A
http://forums.contractoruk.com/
You can create a company online or have the online accountants do this for you.
Good luck!!