I also use TH March and my I pay no where near the price you have been quoted so am surprised at that ! I guess it all depands on the level of cover but I cant imagine that the premium would vary that much ??
A lot of people here recommend hiscox but from memory when I looked they did not offer single insurance for items of jewellery they only offer it as part of contents insurance so other than changing your contents insurance ...
Not much help I know but hopefully someone more knowledgeable will be along soon to help
R