I have a (at the newer end of the range) U.S. built Stanley No. 71 on which the fence screw is U.S. Machine Screw #10-24 - that is, #10, with 24 threads per inch. The fence is factory; the screw's from the hardware store, and is a truss head - larger in diameter than the normal pan head. Stanley tended to be generous with the width on slots for fences and the like, so even the larger truss head is barely enough to bridge the opening; I should get a washer for it.
I don't think the head design matters much functionally, as long as it's large enough to bridge the gap. And a head that requires a screwdriver should be fine; it allows for a good firm setting of the fence.
I have no idea whether there's a translation from U.S. Machine Screw standards to something available in the U.K.; and I don't know if Stanley England used a different thread size/pitch. I have a Stanley of England No. 78 that uses the same thread sizes/pitches in its threaded holes as the American versions, including the weird Stanley non-standard sizes.
You might see if the official British sites for the companies (Stanley and Record) show parts availability. Stanley U.S. used to be pretty good, offering parts for a whole range of their tools, including some discontinued tools. That changed when Black and Decker bought or merged with Stanley. But perhaps Stanley England is different. Same problem, perhaps, with Record, which is now part of that same company.
If U.S. standard machine screws are unavailable there in England, and if your intent is to sell these on to people who will use them (as opposed to those who are collecting them), one choice you have is to tap the holes in the base for screws you CAN get there. Since you'll be making new fences, I suspect the collector value would be low anyhow. I mentioned above that the slot through which the screw reaches to attach the fence to the base is wider than needed; that permits you a lot of latitude to drill out the holes and tap for a larger size screw. The tools will no longer be original, but the wood won't care.
