The RRFSO applies almost as it does anywhere else, enforced by the Crown Premises Inspectorate Group. The main aspect of the RRFSO which doesn't apply in prisons etc is the part that essentially stops you locking doors.
You blatantly don't have control of the premises, therefore the person having control becomes the RP. It is up to them to ensure your staff are safe, and even though the doors can be locked, they still have to protect relevant persons (staff and prisoners alike) "so far as is possible".
Article 20 applies more than article 22...
20.—(1) The responsible person must ensure that the employer of any employees from an outside undertaking who are working in or on the premises is provided with comprehensible and relevant information on—
(a)the risks to those employees; and
(b)the preventive and protective measures taken by the responsible person.
(2) The responsible person must ensure that any person working in his undertaking who is not his employee is provided with appropriate instructions and comprehensible and relevant information regarding any risks to that person.