When trawling the net for advice on cleaning the outside of a helmet shell for the purposes of attaching a self-adhesive camera mount, you get quite a few different dos and donts. Rubbing alcohol, denatured alcohol, detergent, water. Even nail varnish remover. In terms of what's available in England, what have you used that will:
a) Definitely not attack the shell.
ii) Not interfere with the sticky pad's glue.
3) Not contravene the usually vague advice given by Shoei, 3M, etc.