Prestige
The Prestige was always an out and out race machine, taking steel to the boundaries of weight shaving with a steel frame. Many of these used for general riding simply broke, as that was never their purpose. Orange withdrew the model for this reason.
They felt this model could be justified, as they had the Clockwork, which was a good deal tougher for general riding and the P7, which was originally designed as Trekking/Expedition frame to carry heavy paniers and pull trailers on long multi day expeditions. Many were ridden around the Planet, across continents etc with no failures, very different from the modern P7.
If you weigh 15 stone and bearing in mind the frame has some age on it, I would seriously pass it on, or take it up with the seller, as it's not a Clockwork or P7 by any means, these weigh in excess of a pound heavier.
IMHO it would not be safe to ride other than very gently on the road.
I only weigh 11st and I would not trust it at all. An early or late Clockwork or P7 would be a far better bet? The "mid" production Clockworks around the C16r era were also a bit light and fragile tho, as they used well butted tubing for a while and suffered the same.
The temptation for builders is always to go lighter to tempt buyers, but this can be folly as only the very skilled top end craftsman can achieve light and strong, which always means very expensive, aka original Bontrager, Yates, Chris Dekerf, Paul Sadoff etc. I think Orange were after a slice of this market, from a UK built frame to appeal to UK riders, but took the boundaries just too far. There are limits with steel, which is where Ti then starts to come into play.