I hadn't thought about that. Back in the days long before 9/11 QRA was invariably there to react to Soviet aircraft in the vicinity of UK airspace (as it still is for Russian aircraft). We normally had good intelligence on the likelihood of a Bear coming close enough to need intercepting, and our Dragonfly (later Tansor) tanker crews only held 3 hours readiness (I could be at home in Kings Lynn as long as I was on the end of a phone) . If a threat was imminent we came up to one hour, and occasionally less. I assume the Brize tankers are now on something like 15 minutes readiness to be able to react so quickly.