Dave. i suspect everyone uses their own personal shade of grey for RN warships.
UNLESS someone has an ICI colour mix or something, then any similar shade of pale grey will do.
Bearing in mind as well, that light grey can appear to be almost white or dark grey depending on the light conditions, sea state, background, artificial light etc etc
I would look at a few photos and choose something close and call it a day. You mioght like to search on scale model forums, they are more likely to be bothered with an exact match.
Ashley