Found this online , can't guarantee how accurate it is.
"The British brassard for the Corps of Royal Military Police is relatively unchanged from that used in World War I when red letters on a dark blue background was used by the Corps of Military Police.
During 1940 the background changed to a black background. It is somewhat smaller than a US brassard.
In 1968 the colors were reversed from red lettering on black felt and is now of red felt material and 3.5 inches in height, with black lettering, 1.5 inches in height."
http://home.mweb.co.za/re/redcap/brassard.htm