For composite company read
possible tax dodge.
A number of composite companies have been set up for this reason and HMRC pay them
special attention.
And as for contracts, they cannot make you sign away your rights in a job contract. It might be a complicated and long document but you get your basic rights regardless of what it might say.
Legally they cannot make you sign away your right to not work more than 48 hours a week as a condition for getting the job in the first place although many companies (and I've had first hand experience of this) will tell you that you have to sign it otherwise they cannot employ you.