Warren if time goes on and the right sized escutcheon doesn’t show up its worth trying plenty of engineering suppliers around you (should be loads ‘up north’ !)as large washers are available in brass and stainless but not many bother to keep them. They usually sell the really Big sizes as singles so price should be ok just for one.
See what you can find and Let me know if you really get stuck, I might have an offcut of brass I could make one from, might do it with 2 and a quarter diameter if not 2 and a half inch. Depends on what Ive actually got here though- never worth buying in bar to make a single.
See how you get on, it’s not the quickest solution to it, but at least it’s an option if you can’t find anything else. Plus you get one sized with thickness to suit, plus the centre bored out to whatever size you need , I could leave it as a blank disc or bore out for the keyhole or whatever.
