Writers are invariably self-employed. You will therefore negotiate and pay fees, rather than wages or salary. You will not be required to deduct National Insurance contributions or PAYE tax.
The industry agreement which should meet your needs is the one negotiated between The Producers Alliance for Cinema and Television (PACT) and The Writers Guild of Great Britain (WGGB).
This agreement sets out minimum fee rates for both commissioned scripts and adaptations, and for scripts written to an already-existing format (eg that of an established soap). It specifies the minimum that should be paid for an outline, first and subsequent drafts, and