Generally to freelance instruct at a wall you will need:
Relevant qualification (RCI, CWI, etc.)
First Aid certificate that validates the above relevant qualification
Insurance
You may also be required to have a recent enhanced DBS check but that's up to the wall.
You will likely also need to pass some kind of Instructor Induction administered by the Lead Instructor/Course Director on site.
You may also (depending on numbers you are instructing) need to book in any session with the wall. This is usually because the walls need a record of anyone climbing on site and have limits on numbers of guests that people can sign in to supervise. If you're taking in 1 or 2 people to instruct then you can probably just sign them in as your guests but more than that and it will likely be recorded in a different fashion.