Reading between the lines here ,are you trying to connect WC to 10 mm supply pipe ,with an isolation valve and a flexi hose ? If so buy a mini isolator from
screwfix ( code 91386 ) it is a compression fitting , 10 mm at one end 15 mm at the other. The flexi hose goes onto WC valve ,a short piece of 15 mm copper used to join the flexi to the isolator 15 mm end,and the 10 mm end direct to the 10 mm supply pipe.