There is no rule to prevent a flight deck visit once the flight is complete, however, at that point the crew will be keen to get close the aircraft down and get away, so any visit will be a fleeting one. On my last two trips we have had passengers briefly stick their heads through the door - after having asked, of course - at the end of the flight. So, Jinkster, all you have to do is ask - and hope you meet a co-operative cabin crew member who will pass on your request.
Mr 777, I think that you have misunderstood the question - Jinkster wasn't asking for an in-flight visit or a jumpseat ride. Also, the policy re staff using the jumpseats on flights that neither originate nor terminate in the USA is still under discussion with the CAA - not that would help you as an ex-VS man, I'm afraid.