how do I improve the hot/cold mix on my Mira Supreme shower and maintain pr?

depends on where you get your hot water from, the cold will be at mains, if hot from a tank, can only be at the pressure given by the head of water above it, So, header tank in loft, shower in bathroom, about 3-4 ft below tank, so max pressure will only ever be that 3-4 ft of water.
You can get a pump fitted inline to increase the water pressure from the hot, Not cheap. Or get your cold water fed from a tank at same hieght, But that would reduce overall pressure. Other than that depends on how you heat your water. Modern combis heat the mains pressure water, without a tank, so same pressure as cold.

SD
March 2012