At the very least, you need to know what methods contractors and freelancers can use to get paid as well as how you are affected by the Conduct of Employment Agencies and Employment Businesses Regulations 2003, which came into force in April 2004 and updated how the UK recruitment industry is regulated.
This section can't substitute for professional legal advice, but it is a good starting point for any queries you may have and will help build up your knowledge of the legislation that affects you.
More on Legal Issues
Payment options available to contractors
The Conduct of Emp Agencies and Emp Businesses Regulations 2003
Eligibility to work in the UK
Money Laundering Regulations - ID verification
Health and Safety in the workplace
Checking application details and references