I think choosing a street chock-full of high level Soviet functionaries might have been genius. How many of them would the MI6 agent "interact" with during a walk down the street? Would opening the Mars bar wrapper as he passes some general be the signal? Or the bite in front of the bakery? Or disposing the wrapper? Or the cough as the agent crosses paths with a Soviet diplomat? That's a very high noise floor. I suspect counter-intelligence would never figure it out until long after exfiltration was completed, unless they knew exactly what the signal was.