If you're 16-18, and a UK citizen, all you need for enrolment are the following two:

Proof of qualifications, including exam result slips or certificates, including English and maths.You must also bring one of the following that applies:
Any form of ID, such as a UK Passport or UK Drivers Licence.

If you're 16-18, and NOT a UK citizen, you will need the following which applies to you, as well as Proof of Qualifications (if applicable):

eVisa documentation, including passport/visa of family member if your visa states you are joining someone else. We must also see your visa stating the name of the person you are joining.
EU Settlement scheme for EU citizens.
Home Office letter stating that your application is being processed if your current visa is expiring. If your visa expires before the end of your course, you may not be able to fully enrol or complete your course.
We must also see your national insurance number if you are 19+ unless you are an asylum seeker. You will be charged full cost of the course if you cannot provide an NI number.
If you don’t bring the documents relevant to you then we may not be able to complete your enrolment.

If you are an adult learner (19+) you will also need to bring:

Payment method to pay for any fees due (we do not accept cheques or cash).
If you live in a London postcode area and are earning below £27,007.50, or live outside of London and earn below £25,750.00, we will need to see wage slips or your employment contract stating your annual salary.