What king of brittish car do you have?
I had a 62 xke, 3 su carbs, 9.5 compression ratio. It was designed to run on 98 octane. I would get pinning and dieseling on 93 octane. The only way to prevent that I had to retard the spark. You may try using a colder spark plug. The hotter the plug, the more it acts like a glow plug
I finally sold car xke years ago.
Recently I purchased an exhaust gas temp gauge. I leaned carbs on Jubilee and IH C past the point of max temp. Running the engine a little cooler. That may solve the dieseling, but the side draft brittish carbs are touchy. Best way would be to get pistions and heads with lower compression.
Another way would be to shut off the electric fuel pump and let carbs run out of gas. MY xke had electric fuel pump.