By the look of it and by the size given by you it points towards a Saduria (Mesidothaea) entomon. This is however an arctic species (whis is a glacial relict in the Baltic Sea, and also in some freshwater lakes like Lake Vänern and Lake Ladoga). I have no idea however about British recordings of this species (I'm from Sweden).
Thanks. It looks like you are spot on! I compared with pictures of a live Saduria entomon and it checks out perfectly. The dried sprats came from Germany, so they were probably collected in the Baltic.
I decided not to feed this specimen to my dog with the sprats!