I thought only the USA & Canada needed shims and the UK bikes got adjustable needles like the rest of the world.
FIVE shims per carb? What brings you to that conclusion? Most folks that shim don't need more than one or two per.
Since I've never been to the UK, I don't know where you'd go to find them, but you can try taking a needle and a slide down to a store that sells electronic parts or a place that deals with RC planes/cars/etc and find a suitable washer with an ID that fits around the needle and an OD that does not hang up on the wall of the slide when you put the needle back in.