Marrakai,

Here's a photo of an original MacNaughton round action double rifle in .303 that has an aperture sight. In fact, it has no other rear sight, the rib being designed entirely around the aperture sight.

This rifle is late pre-WW2. Last fall it was under the ownership of Graham Mackinlay. I had a good deal of conversation with him about it at the Vintagers, but I didn't get to see the rifle as it remained in Scotland.

Another British rifle wearing an aperture sight is the .500/.465 NE Purdey that just sold off Westley Richards website. It has what appears to be an add-on folding peep sight mounted on the toplever. The rifle also has a quarter rib with express sights:

Whether this is original to the gun I have no clue.

That's about all I can point to on the British side. As you certainly know, the continentals came up with all sorts of tang-mounted folding aperture sights. I am assuming your inquiry doesn't include those.

Hope this helps,
Curl

P.S. I have another, relatively poor photo of the MacNaughton sight from directly above and behind. This reveals much more detail of the sight assembly. I'll post it later today when I have access to my FTP.