Its straight forward...
Like others have said, you need a CIS (Construction industry scheme) registration, as well as a UTR, the UTR is just a reference number. When you do work for anyone as a subcontractor they deduct tax at 40% from you and give you a CIS voucher which you keep, and you pay your own NI quarterly. When your accountant tallys up your expenses at your year end, such as tools, work wear, equipment, phone, use of office etc...He works it against all the CIS vouchers you have collected during the year.
The 40% tax the contractor takes from you and pays on your behalf i