There seem to be at least 3 mints and various sizes and finishes. These notes from BCS 1 N#490017
"Issued by The Britannia Commemorative Society, Philadelphia, PA & The Britannia Commemorative Society Ltd, London.
This medal was the first in the series of sixty by the society. The silver 44 mm medals in a proof finish, were issued only to members. The Franklin Mint struck 1395 of the silver 44 mm medals for distribution to the society's North American members, the rest (809) were struck by Johnson Matthey & Co. (reference?) for other (England & World) members.
According to page 8 of an article on Dora De Pedery Hunt by Brian Thomson, in The Canadian Centennial Collector, Summer 2015, Volume 1, Number 4 (which also describes BCS 4).
"The Britannia Commemorative Society is devoted to British history. The dies are made at
The Franklin Mint and the 1395 fine silver proofs are struck for each one of its members in
North America before the dies being sent to Great Britain where 809 proofs are struck for
members in England and other parts of the world."
In addition, other sizes and metals were sold publicly. Bronze, Silver, Gold, & Platinum 57 mm are in matte finish with limited but unknown mintages. John Pinches Medallist Ltd., London struck all the 57 mm medals (reference ?)."
It would seem Pinches didn't issue 44mm medals?