Those wrenches are fairly common in the U.K.
The exact style varied slightly over the years, but the quality was decent.
The part number (E.G. RJ600) means that the hex size is .600”. Thus both Whitworth snd SAE wrenches have part numbers starting with “RJ” and only the number changes. Metric wrenches are “RJM”.
I quite like these wrenches to use (especially this exact pattern), and have complete sets from 3/8” to 1 1/4” A/F and a comparable range in Whitworth.
The metric sizes are hard to find as Britool didn’t sell a lot of metric in the U.K. back then. Nearest modern wrench in style and quality (and almost dimensionally identical) are the Hazet 600.
Britool were bought by Facom in ‘92, and they used the name on tools of standard Facom pattern, but with a poorer finish, and possibly inferior steel.
The name lived on when SBD bought Facom, and they marketed their “Expert” tools as “Britool Expert” in the U.K. but given that the British factory was long gone, and the tools were shite, that was a bit of an insult really!