UNF and UNC are only used in USA and Canada, not sure why you think we would have them here?
We import machines and cars from all over the world, but not many from USA and everyone else uses metric. We still have some GM cars, but they are European models, made in Europe by European companies GM once bought up. There are a few Jeeps, but otherwise it is just the odd classic for USA built cars. There is of course a market for non-metric for classic cars, but they mainly use British sizes, not USA sizes.
The tripod mount was a British standard originally, the USA version has a slightly different shape thread but is compatible, so there is a possibility that a classic car specialist will have something suitable.
All the sizes, metric, British, USA are easily available online in the UK, but a hardware store, even a car based one will only have metric, plus a few other sizes that for one reason or another are still common. Garden taps for example are still measured in inches, but only the garden ones.
Billion of dollars worth of industrial, farm and power equipment is exported from the US and Canada to the UK each year and these machines certainly require parts and servicing.
As it happens,
American cars are fairly popular in the UK, especially trucks. The Ford F-series is one of the most popular but also cars and SUVs such as the Ford Mustang, Cadillac Escalade, Chevrolet,
Dodge, GMC, etc.
Classic American cars are also in demand in the UK. While
these vehicles may only represent a small percentage of the vehicles on the road they do indeed need parts and serving too. This was the reason I used the term "appropriate dealership" when I suggested them as a source of UNC and UNF bolts & nuts. Obviously, anyone owning any of these brands will require parts sooner or later and the law of supply and demand suggests that there are places they or their garage mechanics go to purchase them.
It's true that various vintage British vehicles and some vintage farm equipment use compatible bolts, nuts and washers and there are many
suppliers of such parts. And despite your claim these are indeed offered as UNF parts using American fractional sizes. You don't see too much in the availability of BSW, BA or BSF these days but they too are
available.
Practically every
UK supplier of bolts, screws, washers and fasteners
offers a selection of UNF and
UNC parts.
Originally, I recommended Amazon.UK as a
source for UNC and UNF bolts, nuts, washers etc. and this would certainly be a convenient place to look for such things for a small project like this, so, once again, your pettiness in initiating another one of your petty spats over a subject like this is beyond bizarre.
As for tripod mounts and rigging the 1/4-20 thread is the de facto world wide standard these days and for this reason you'll find numerous products and parts made to accommodate them or modify them especially in the film, television and photography industries. I know from personal professional experience that these industries require a source for the basic bolts and nuts of their trade as this threading is used for wide range of mounting purposes. When you suddenly find yourself needing a part or a piece of hardware on a photo shoot or a cinema project you don't stop production to order online, you send your production assistant out to source the part at one of the many suppliers that service the industry but more often than not it's the local quality hardware store in whatever city or town one happens to be in at the moment. If a metric bolt or UNF/UNC bolt or nut should be required, or any other standard hardware item it would be immediately available.
So, indeed I still find it hard to believe that there is no local availability of such parts in the UK in outlets such as hardware stores or machine shops, but as I said before, if true it would be a real shame, and a sad commentary on the current state of affairs in the UK.
Here in the US one can generally source whatever one wants or needs. I find that to be the case even though I reside in a rural area. It wasn't always the case but the global supply chain has come a long way in recent years. Of course, if you go to a large urban city such as New York, Chicago or Los Angeles for instance, you'd be able to find literally any part, material, food ingredient or other esoteric item from all over the entire planet. Perhaps the provincial UK sentiment you espouse here, as you do so often is in line with the whole Brexit philosophy of isolationism you champion?
That is the main advantage of metric; with just 6 metric wrench and socket sizes you could do 75% or the work (and almost all of the maintenance work) on imports, but US cars have always required a full set of whatever kind 
