I would think these are all the same thread so the ones from the other unicycle should fit. I took the bolts out of a bike cotter less crank yesterday to have a look and they are quite fine threaded (which is why I mentioned checking to see it the bolts you had weren’t too coarse).
Given you’ve got another unicycle to try this on you can get an idea of what it should feel like on that. On your unicycle try to get the bolts to go in without the cranks on, you will see what is happening a lot better. Given you’ve got both M8 fine and coarse threaded bolts you will see there is a pretty noticeable difference between the threads, you should be able to see what the thread looks like in the hub to see which one looks correct – I would think that it is the fine one (M8x1)
I’m assuming you had to get this posted to you and the seller removed the cranks but didn’t send you the bolts? That seems a bit suspicious. You mentioned M8x1.25 bolts, are these just “standard” M8 bolts you got from shop – ie not specific cotterless crank bolts? “Standard” M8 bolts for just general bolting things together will probably not fit and if anyone has tried to force these in they may have damaged the end of the thread in the hole. In that case you can probably fix it by running a tap down it as mentioned above – take it to a bike shop to see if they can do that for you (I’m assuming these things aren’t heat treated/hardend and that they can be retapped easily). Even if the thread is damaged on the end at both sides the good news is that the damage should only be at the end and there should be enough good thread left for the bolts to easily take hold.
There is always the possibility that these aren’t metric threaded, but I doubt that. You could email or phone UDC in the UK and ask them, I would think these are all pretty standard though.