ATCO1987,
As you correctly guessed, the answer is no. The CAA has issued a list with categories of people who are allowed on the flight deck during flight. At this moment a security agent is not on the list, so you are not allowed on the flight deck during flight.
However, the CAA can agree on additional categories but you must have a reason for being on the flight deck. So unfortunately, i give you no chance to get on the flight deck during flight.
FT