I read only part of one of the articles Dan cites --
https://bicycles.stackexchange.com/questions/10236/is-reversing-the-stem-pointing-it-backwards-a-safe-way-to-reduce-reach-length
-- before starting to wonder about the teachers who hadn't taught the contributors to put their minds in gear.
There are only four facts that matter here.
1. A properly designed bike, fit for purpose, is a stable machine when in motion. As an example of the amazing dynamic stability of a bicycle with proper geometry, an English scientist*, bored out of his skull in an office job after the excitements of WW2, proved for all time in the parking lot that a bicycle with the handlebars, and consequently the steering and fork,
reversed is dynamically stable when pushed and then allowed to run until it ran out of steam and fell over. (Not with my bike you don't, not even in the Service of Science!) I expect that a Thorn bike of any generation would be competently stable, but in any case we know for a fact from a chorus of experienced users that the Thorn bikes of the Andy Blance era were conservatively designed and thus extremely stable.
2. There is no mechanical difference in operation between a forward-pointing stem and a rearward-pointing stem except, for the dementedly obsessive, the theoretical fractional surplus of stiction between tyres and road in pushing rather than pulling, and, since one hand is always pushing and the other hand pulling when a cyclist turns his bike, these fractional forces balance each other. All that matters is how long the lever arm is: the lever arm is the direct distance between the cyclist's hand on the grip and the pivot in the centre of the steering tube; Dan has already pointed this out. The 'twitchy bike' in the discussion group that irritated me is mere confirmation bias, if a real fear, or if not, wannabe experts posturing.
3. There was a time in the early history of the bicycle when tiller type steering was common, then less common because wheelbases were short and knee-room had to be found somehow. Tiller type steering of various degrees of sophistication still has a foothold in recliner bikes, where they are a vast improvement over the only alternative, rear wheel steering. If you want to feel real instability, try rear wheel steering in a tadpole recliner tricycle; every move is a sudden-death playoff, a lose-lose event. (In automobile suspensions the concepts of leading and trailing positional or reacting lever arms has validity but only because so much more power is in play, and, as can be seen in any Citroen or modern hatchback rear suspension, the engineers never agreed on which way is the best compromise.)
4. When I made my truck assisted personal record ton-up downhill bicycle ride, I used an adjustable stem bent so far down that I put a piece of thin foam between it and the head tube to protect the paint; in the stem the North Road bars were rotated so that the grips pointed nearly vertically downwards beside the fork legs to get my back flat. In short, there was virtually no 'stem length' and any input should in theory have resulted in a noticeably more abrupt steering response than the normal settings. I can assure you that at a 100kph and some small change I didn't fancy chewing tarmac, and was paying strict attention to the bike's responses. This is an anecdote with a sample of one, so I'm not claiming the status of science or history here, but in racing my specialty was suspensions and I was my own test driver, so I know what to look for.
I therefore think Binky can save his money and to start with merely turn the stem 180 degrees to point backwards. He might, depending on his and the bike's precise measurements, after his initial tests have to buy a shorter stem, or possibly (but not really likely because knee room will be critical), a longer stem. And it would be smart at first to ride cautiously but that's pure common sense.
One more point. I like wider grips-spacing than is fashionable among the drop bar crowd. Binky might consider North Road bars. They're extremely ergonomic used right way up, they can be flipped over front to back to become bullhorns, and they can be flipped to become mustache bars, all of which means he can fine-tune his fit on the bike, his reach, and his back angle almost infinitesimally, and even more so when partnering the North Road bars with an adjustable stem. North Road bars are cheap, or sometimes free from the sort of LBS which makes its main profit from selling 'upgrades' to the unsuitable bikes they have persuaded people buying bikes on government schemes would give them a 'sporting look'; one of the first things they throw off are 'old fashioned' North Road bars.
*AI is outrageously useless. I asked Siri on my Mac with the best Apple AI, 'What is the name of the Englishman who proved that a bicycle with the handlebars and all attached to them turned around is nonetheless stable if pushed to speed and then allowed to run without a cyclist aboard?' It replied, 'An Englishman is a man from England. It can also refer to several specific people, such as the Canadian rock singer Jason Englishman or the reggae musician Erald Briscoe, who records under that name.' Hey, Apple, did you intend to insult me with pop trivia? Shame on you.
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