Out of curiosity, I hunted in my “hope chest” for some long Allen screws. Now most of these screws are pretty old and a mixed bunch so I cannot speak for more modern examples that may not have even been made in the UK but I found that the plain portion of these examples to conform quite closely to the nominal thread size. From their appearance I am sure they have rolled threads so it would seem that British manufacturers at least took the trouble to maintain the nominal diameter on the plain portion and reduce the diameter of the length to be thread rolled.
This might not hold true for more modern products there is after all the attraction to manufacturers of reduced manufacturing and material costs, not much on one screw maybe but if you are making thousands . . .
The only answer is take a micrometer with you and measure before you pay!
As far as using 10mm screws is concerned I see no reason why not as long as the head of the screw can be accommodated. Indeed an increase in the area under the head might be an advantage for load spreading when the screw is tightened down particularly as Allen screws tend to have a smaller area here than the equivalent hexagon head and washer.
There is little to be feared by Mr. Average “man-in-shed-with-lathe” about turning down the shank of a long screw. It only requires that the end of the screw be centre drilled, a centre in the tailstock can then safely support the end of the screw. From there on it is only a matter plain turning and anybody that can cut a halfway decent thread in a lathe should have no problem doing that.
When it come to thread cutting in the lathe by the less experienced I’d recommend cutting the thread to around 70% of the full depth and finishing with a die because: (1) the cutting tool does not need to be perfectly accurate (2) a partial thread will guide the die true, you don’t want a drunken thread on a long bolt (3) removing the bulk of the unneeded metal reduces the strain on the cutting edges of the die and improves finish.
G’on, ‘ave a go, ‘yer know you want to! 😉