In answer to the original question...
EU OPS 1.735
"An operator shall not operate an aeroplane unless the following equipment is installed:
(a) In an aeroplane with a maximum approved passenger seating configuration of more than 19 passengers, a door between the passenger compartment and the flight deck compartment with a placard ‘crew only’ and a locking means to prevent passengers from opening it without the permission of a member of the flight crew"
UK CAA FODCOM 21/10
"For Commercial Air Transport and Public Transport operations being conducted under EU-OPS, JAR-OPS 3 or the ANO, no person may be carried on the flight deck except a crew member assigned to the flight as an operating crew member, or a passenger permitted to occupy a flight deck seat in accordance with instructions in the operator’s operations manual."