[Mike]

Never found W1A funny. 

Allegedly the reason the building tours stopped was because undercover police booked on a tour, left suspicious packages during the tour, and found that no one challenged them and, at the end, they were all still there.

It won't have been the police doing that, more likely the BBC themselves paid someone to do it as part of their building penetration testing regime or on advice of a police CTSA. Transec as was, did this for the railway, airports and maritime installations.