I own one of the (according to terry mcgee) rare keith, prowse flutes.
if I were to insure it, what value would I need to put on it? I have no clue.
Not sure if this is of any help but a Keith, Prowse sold for 920 sterling on E-bay, March. 2011
http://www.ebay.co.uk/itm/antique-wood-wooden-flute-Keith-Prowse-London-/260749206659
Keith, Prowse & Co flutes are discussed here on the concertina.net forum apparantly Liddle and Wylde were later involved in making flutes for Keith, Prowse & Co. Earlier Keith, Prowse & Co seemed to have been made by Thomas & Jospeh Prowse? They are considered very good instruments, but you already know that
http://www.concertina.net/forums/index.php?showtopic=4705