As a runner most of your jobs will be PAYE. You’ll be paid through the company payroll system and Tax and National Insurance will be deducted before you get the money.
But there are occasions where an employer can ask you to send an invoice, when a job qualifies under the ‘7 Day Rule’ which is peculiar to our industry. This where an engagement is for six consecutive days or fewer (including any days off). NI should still be deducted but you will need to pay any tax owing through self-assessment.
BUT (and please remember this) you DON’T have to register self employed. You are not becoming self-employed, you are merely declaring some additional income on a self assessment form.
Here is a simple template for an invoice you can download free if you need one (shared on Google Docs with no login required or data collected).