A director is a person appointed by the company members to run the company on their behalf. A director can act as both director and secretary. You must be registered as a director of the company with Companies House.
As a director you can't be classed as self-employed.
Don't fill in this form if you want to register as a director, fill in form SA1 (Registering for Self Assessment and getting a tax return). Follow the link below (on the GOV.UK website) to complete form SA1.
SA1 - Register for Self Assessment and getting a tax return