Brexit to blame for falling BSA values? That must be the best one yet... 
Thats not what I said or meant, But I dont want to destract from this guys bike, which is well priced and appears nicely done, since it would be challenging (At least here in the US) to restore one to that level for the average enthusiast for that price.
So 2 different enchiladas here,, ones chicken & the other shredded beef. (Maybe a Pork Carnita as well)
First, BSA Prices. Certain models are more valued in specific markets. Might have been the ratio of exports/imports, age of the machine and model. For example 350cc bikes were never big sellers in the US. Visit a US Norton club,, majority of the bikes are commandos. So when it comes to BSA, Goldstars and compy models have always commanded premium dollars and yet,, many of those are dropping. But I know of several land fill sites full of grey porridge BSAs that were dumped in a hole in the ground because you couldnt even give them away back in the 1980s and the scrap yards would not take them without titles. Anyone wants to dig them up, I can point it out.
I used to run a shop specializing in British bikes and I often built bikes to sell. I couldnt even get a return on costs to restore an A65. Same with Nortons.. A 1960s BSA A65 compared to a 1960s Triumph was basically 1/3rd the sales price, plus the A65 cost more to restore with all that chrome.
So I have always advised newbies and cost concious British bike enthusiasts the best value for the money has always been BSA for a rider or for fun. Not for investment.
For the more desirable models, (Hornet, spitfire or a Goldie) unless you get one very complete and at a good price, you will exceed value by trying to buy piece by piece. My old guru was GoldStar Ron Halem (RIP) and he said you could S**T in a box, write Goldstar on it, put it on ebay and it would sell.
So, last 2-3 years Triumph numbers have gotten very soft. I know many who build restos to sell (Many at Las Vegas in the big auction) and prices are dropping. BSA gotten softer as well. Norton Ironically has climbed a bit overall.
Now I know a number of people with early BSAs,, singles and twins and theres just not the big numbers of people into them here in the US. The demand for them has always been driven by overseas markets.
So, second enchilada, What are the factors why? I agree that there is a shift away from hands on vehicle repair, restoration and collecting by some. But overall its not the driving factor. I try to attend as many shows as possible and if anything vintage vehicles are more popular than ever and its the 20-30 somethings driving it. But several factors there. They dont have as much money.
Typical bike marketing had 2 demographics. Young riders coming up from learners to daily riders and a bit of racing. Then families and work kills it for a decade or 2, then 2nd childhood you bought what you lusted after and many customers in 40s-50s and early 60s recreating their youth or passions. So the bikes they grew up with (or cars) were a target.
That seems to be going away.
In the US there is a underclass, and a above average income class,, middle class is dying. THAT is whats impacting the motorcycle and collector car market. Age wise,, I know some 20 something guys in 6 figure incomes, and they buy what they like and believe it or not,, its collector bikes and cars but its what they perceive as cool. Not always restored. Period choppers, period race bikes and memorabilia is whats selling.
While classic British is doing okay,, come to a show like the ONE moto here in Portland, Mama tried, Or "ShowClass" in SoCal. These shows are HUGE! and the market is 20-40s in age. BMW has sent most of their design and marketing teams to the ONE show to figure out HOW to market to these people. HD Knuckles, Pans are the holy grail. Pieces of Junk Iron head sportsters are popular again. Shovel heads are popular as well. The guys that built this bike? Late 20s and part of a very active bike building scene.
See:
https://www.ebay.com/itm/1966-Triumph-T100-/264451418954?nma=true&si=MrOg9rDOVLp%252FH%252F5ZS3Fjy3YJjmA%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557The tastes change and ebb & flow but its not going away. Its more nuanced than that. Markets peaked for collector and vintage vehicles in mid 70s, late 1980s, up and down in 1990s, crashed in early 2000s, went insane up to 2008, and been volatile ever since. I still have a Classic Bike article bemoaning in the 1980s one of the first 1960s Bonnies selling for $5000 and saying it was the end of the hobby as only rich people could afford them. For a while I was getting $15,000 for a 68-70 Bonnie, but now, you are lucky to get $10k
So what are the outside influences dictating US markets? Its generally tied to global financial markets. There was a period that the Australian dollar dropped and a ton of bikes imported into the US from Aus in early 2000s. I have 2 of them, and missed out on some others I couldnt raise cash fast enough for. Conversely,, one of the Forum members here.. Mike Reilly who I know, has made a career out of exporting British bikes from the US into Australia. As in Shipping containers full.
Right now, Brexit or other factors,, I dont know for sure, but the English pound is the lowest its been against the US dollar in decades. I know of a lot of UK wheeler dealers who troll the US and export back to the UK Asian, British and European bikes by the container load. That market has died down,, most are in wait and see mode, but its not financially feasible. I know several Europeans doing the same,, The Canadians are also taking a hit,, Look at the exchange rates. Right now buying parts from Canadian suppliers is like a 30% off sale every day. (Walridge, BCS, etc)
So a bike like an A10/A7? They are more valuable outside the US generally, But if the main markets for them like Australia, NZ, UK and Europe are suffering from currency issues, much of the demand here in the US evaporates.
So is Brexit an issue? It has to be to some degree, but not entirely. But its inescapable that the UK pound is not doing well. Right now its 1.23 to the USD and has averaged 1.65.
