Tengah Type I think that you are being very optimistic with your reaction times and having a bunch of sleepy disgruntled armourers at the line 'in a few minutes' is unrealistic. I would say at least 30' from initially rounding them up and that would in itself be remarkable. As to the ready-prepped gunpacks, these would have been loaded with practice rounds i.e. ball and from what I recall from my time at Chivenor and Brawdy only two guns were ever pre-loaded. Even in those days where the words Health & Safety were yet to be thought up, it is inconceivable to me that, in peacetime, gunpacks would have been stored with HE ammunition as a matter of course. However, someone better placed may correct me.
All in all, taking into account the time of day, the chain of command, the ancient comms system and human reaction times, I cannot see a Chivenor Hunter being ready in less than an hour from the first moment that the US called for help even if they did so which is open to question. By this time the horse had well and truly bolted..