The issue you describe sounds very similar to carb icing .
Surrey can have hi humidity in summer.
Icing creates snow down by the throttle plates, - that then plugs the idle circuits in the base plate.. - causing it to stall.
You can immediately remove the air cleaner, - and shine a light down the carb, you very well may see snow on the throttle plates.
Cure, - let it warm up a few mins, before driving .