You can give your snake a bath in some lukewarm - not hot! - water with a drop or two of mild dish soap added. The soap is to decrease the surface tension of the water so it drowns the mites; your snake should not look like it's taking a bubble bath.
After the bath you can wipe your snake with Reptile Spray. If you don't have that in the UK I've heard that people here in the US have used Frontline (active ingredient fipronil) spray successfully. Don't spray it directly on the snake, spray a paper towel and wipe it on the snake from neck to tail. You don't want the snake ingesting either product so use a very small water bowl that the snake can't soak in afterward.
