I agree with the other Michael...Woodhall Spa.
I know it is a stretch to call it a true Hidden Gem as it is perenially ranked highly, but in terms of actual play it is not high on most American's list when they travel to the UK.
A great trip is to play Woodhall then move up or down the coast...down to Brancaster, Sheringham, Hunstanton and Cromer or up the coast to play Ganton.
Myself...I have a tee time on July 5th at 9.05 at Woodhall, on my lonesome...would welcome any company?