I have not recovered any A10 seats but done loads of Unit A65 and Unit Triumphs. Its a mixed bag and depends a great deal on what you have (Condition and style) and how close to stock you want it to appear.
For 100 point show bikes, clearly the std is much different, but a close approximation and comfort,, well that too. The marketplace has changed a bit since I was doing a lot of shop work. I used to rebuild seats as a matter of course and kept a number of them in stock, rebuilt as well as new replacements.
For rebuilding I would strip it down, sandblast and treat with chemicals (Rust treatments) then paint the base both sides with a single stage Poly. I then used new foam and covers. But the cost of the foam has quadrupled while the seat covers have stayed relatively affordable. (my wholesale was around $35.00 per seat cover) but the foam was around $85 last time I ordered. This is preformed and ready to use.
I can outline HOW to do a seat, but it takes practice and after 3-4 you can get nice results. Getting it just tight enough and not crooked takes some thought and practice.
Used to be wholesalers would run sales on seats for shops and bulk buys got you a further discount. Even British Only (The most expensive US Stockist) Had decent deals and tended to pay around $130 per seat ready to install. So, was rather affordable but they were primarily more popular models and harder to source less common models. because of the rise in seat foam I did fewer and fewer rebuilds of seats.
Now, on some bikes and show quality bikes its the little details that make the difference, for example on certain Unit Triumphs the original pattern seats had tiny diamonds in the top cover,,,But the repops have little circles. Only a truly OCD rivet counter would ever notice and I believe its the same company who make the covers both original and repop. I know of anally retentive bolt polishers who actually took a used top section and had it sewn into a repop side skirt just to get that pattern.
Foam tends to be a problem because in most cases it turns to dry crumbly powder and every ride showers the rest of the bike with a pollen like powder. Foam does not last forever, especially when exposed to battery and oil fumes.
As noted, there IS a wide variety of foam out there and if making your own pads,, the sky is the limit. Some upholsterers use 2 layers, a denser memory foam on the bottom and the cushiony open cell foam on top. I have paid some local upholstery guys to make some custom seats where I supplied the bases and a drawing, and pretty nice but not cheap. In the US I am a big fan of the Corbin gunfighter seats on some of my other bikes. (Sportster, Norton, Buell and Ducati) But one of the most comfortable seats for touring is a company called Russell Daylong seats.
Nothing for BSA but might get some good ideas.. I rode a few bikes with them and I think they are ugly, but damn they are cushy for the tushy!
See:
https://day-long.com/While the corbins are fairly dense material all of mine are very comfortable except one which I sold,, It was a corbin sundowner and even short rides it hurt like hell,, but I really like the rest of theirs. (Again, none for BSA)
I once built a cafe bike for an old guy, and he wanted lots of padding, The seat pad was physically taller but once sitting on it looked fine (Weighed down) and some people made comments, but he didnt care, at his age just riding a cafe bike was a big accomplishment so who cares what it takes.. But we used several types of material a upholstery shop came up with. Early BSA and Triumph seats were pretty much a plank with minimal cushion, so lots of room for improvement.
Here is a blog with a guy who obsesses over the small details and discusses the seat issue, In an older blog he rants about the seat issues quite a bit. (And is one of the people I know who will sew together old seat covers with new parts). Sadly, once done he never rides the things. All his bikes have zero miles. Museum ornaments.
See:
http://triumphmotorcycles.typepad.com/my_weblog/2014/03/leroy-turner-starts-the-restoration-of-a-1967-tr6c-for-our-collection-part-3-of-a-multi-part-blog-the-front-end-r.html