White backed and say made in England by any chance??? or Turkey???
My money would be on the Johnsons PW1 8x6 (Polar white) but it was a very popular size back in the day and loadza companies made it. Original size of the PW1 was exactly 200x150, but they started making them smaller around 2000ish so that might be what you have. Since been discontinued as has that size by nearly all manufacturers everywhere.
Best you can do is get hold of some of their PRG1 (prismatic gloss white) in 197x197 (closest size I can think of) and wet-cut them down to size. Its probably all you can do now.
