Jumpseat access clarifacation
Jumpseats aren't stopped!
ATCOs are allowed on the flightdeck of UK registered carriers providing Captain has authority in advance from airline ops.
At Heathrow we have access to BMI, BA, Lingus and Virgin.
With regards to non UK operators, I know from first and second hand that ATCOs can get flight deck access ad hoc if a passenger on a flight, Lufty, Air France and Ali are all very flexible with regard to this.