Previously and possibly 20+ years ago a hijacked plane was supposed to fly to Paris at the request of the guy with the gun. Instead it landed at a well disguised Stansted bedecked in French flags and signage and I believe also with staff dressed in French Gendarme Uniforms. The ruse worked really well and the hijacker did not realise anything was wrong until he was arrested
Harsh as it might seem it is sometimes in the best interests not to alert everyone to the fact that measures are being taken to deal with a security situation.
As Stansted was nearer than LHR when the instruction to divert was received I would imagine the crew were a tad busy reprogramming the systems, rummaging through charts for STN and making radio calls.