For my list of ‘false positives’, this is 40ft Royal Barge built by Vospers running trials at Portsmouth (1938). This 24 knots craft served as a tender for the Royal yacht Victoria and Albert. This craft has been spotted aboard the Royal Yacht Britannia as well but appears to have been replaced by another 40ft craft.
Cane, du, P., High-speed small craft, 1964, Temple Press Books.
THE KING WITH THE INVASION FLEET. 24 MAY 1944, HMS BULOLO, AT BEAULIEU ROADS. THE KING VISITED MEN OF THE INVASION FLEET ON BOARD THE HEADQUARTERS SHIP HMS BULOLO. © IWM (A 23599)