Not last time it wasn’t. I’ve had the dubious pleasure of being held up by AF1 and the war machine (they travel in pairs) on a couple of occasions in the U.K, once at LHR, in exactly the scenario you describe as not happening. On the first occasion in particular (UK but not LHR), it resulted in a number of diversions. They have their own way of operating (I seem to remember they informed ATC what they were doing and just did it on the first occasion).
My understanding is that when President O visited, he used Stansted as the chaos at LHR by President GWB was well documented.