I have finally managed to purchase an infamous RAF 'ditching' whistle for a reasonable price (less than £20! I have never understood why, given the quantity available, they command such a high price).
They were often attached to the collar of the battledress blouse. I have attached some wartime pictures showing the whistles being worn, interestingly they appear to have been attached to both sides of the collar. The right side of the collar has the 'hook' and the left the 'eye' or 'loop'. I would have thought that attaching the whistle to the 'hook' meant that you ran the risk of loosing it when bailing out.