I'd agree with you regarding the stuck-on panes getting irritating the more you look at them
- effective perhaps for an hotel or similar, but in an intimate domestic setting they're perhaps overly obvious. This is possibly how Wynns as a protected building got away with it - a city centre hotel with all the attendant urban noise intruding on guest bedrooms could have been looked at with an element of sympathy by planners. And the sashes were 'only' 80 years old, and probably had modern glass.
Given how relatively new this product is to the market, I've no idea how planners view its use in protected structures, cobalt - others would be better placed to answer. If I was in your position, I'd probably go out of my way to retain as much of the original frames and glass as possible, but failing that, reproduction sashes with high performance single glazing is definitely the best option in fulfilling both aesthetic and most insulation requirements. No idea as to cost for such glass.
But again the old chestnut raises its head - what exactly is 'energy efficient'? That is, if you don't have room-by-rrom themostatically controlled heating, having doubling glazing isn't going to make the slightest bit of difference either to your consumption of energy or your fuel bills. If your heating is turned on and left on as it is in most people's homes without constantly sampling room temperature and turning off accordingly, double glazing, and indeed arguably any insultation, is as good as redundant insofar as fuel consumption is concerned.
Not that this should in any way be used as an excuse in older buildings - as every building should have room thermostats - but it's also a simple reality that most properties do not. And even if you do, well heating is generally put on in the evening, when shutters are closed and curtains are drawn. The generally modest sash window is by no means the elephant in the room as far as insulation is concerned, unlike walls and ceilings, and they even feature shutters unlike expansive 70s picture windows.
Just some points to consider...
Secondary internal glazing is also an option, and can work well in some cases. But generally I've yet to see a good example for Georgian or Victorian sashes (casements adopt it much better). With the former, it also tends to ruin the architecture of the internal sash and shutter ensemble