EU OPS 1.1235 states:-
"An operator shall ensure that all appropriate personnel are familiar, and comply, with the relevant requirements of the national security programmes of the State of the operator."
and 1.1255(c) states:
"In all aeroplanes which are equipped with a flight crew compartment door in accordance with subparagraph (b):
1. this door shall be closed prior to engine start for take-off and will be locked when required by security procedure or the commander, until engine shut down after landing, except when deemed necessary for authorised persons to access or egress in compliance with National Aviation Security Programme;"
There should be a procedure in your OMA Section 10 which is accepted by the CAA.
The NASP in the UK is a restricted document, usually the Operator's Aviation Security Manager has a copy.