I've been on a horse when it bolted - I was riding in a small group and it was one of the others which spooked (still don't know why) and all the horses shot off together - it was old, old instinct of a prey animal in full flight and sticking with its herd. Just running as hard as it possibly could, covering ground as fast as possible; tunnel vision - not caring where it was going just as long as it put the perceived danger as far behind it as quickly as possible.
I was on a pony that was fairly armchair/schoolmaster type, but I have never felt such power underneath me before or since (and I've ridden strong ex-racehorses and flighty ex-polo ponies).
You cannot do anything about it. Nothing. Just try to stay on board. In the first madness, you can't even steer. But they will run out of steam, and as a rider you will feel that happen (you can begin to steer, pace drops to normal gallop, then you can start getting it back). How long they can go in the first uncontrollable bolt depends only on how fit the horse is.
I am astounded at 5 miles across central London. But yes, after a distance I would expect the adrenaline rush to be abating, the horse to become more aware of its surroundings, and to become catchable by someone who knows what they're doing. (Emphasis on the last few words - an injured horse can be difficult to handle, even if otherwise exhausted)