Entering Miami was a breeze. However, I have Global Entry and didn't check my bag.
Leaving San Francisco was also a breeze. Global Entry gives you TSA Precheck. Best thing ever.
Arriving into LHR T3, again, very easy. Admittedly, I'd clocked the other inbound aircraft and wasted no time faffing around/dawdling. Having the priviledge of being in Virgin Upper class meant I was off before most of the passengers. The e-gates were working well and again, no checked bag meant I was out of the place in minutes.