Agencies and Companies that use Contractors are reluctant to pay you for your time unless they receive a valid tax invoice. This is because if they are deemed to have paid someone who should have been an employee then they can be penalised for the PAYE and National Insurance you should have paid if you had been an employee.  You are therefore required to either set-up your own Limited Company or become part of an Umbrella Scheme (which clarifies your employment status).  By joining Umbrella you become in effect an employee of Umbrella which clarifies your status.