I found this on the usps site. Have you taken it to usps already and they said no?
Country Conditions for Mailing - Great Britain and Northern Ireland
(Includes England, Scotland, Wales,
Northern Ireland, Guernsey, Jersey,
Alderney, Sark, and the Isle of Man)
Coins; banknotes; currency notes (paper money); securities payable to bearer; traveler's checks; manufactured and unmanufactured platinum, gold, silver; precious stones; jewelry; and other valuable articles, may only be sent in registered letter-post shipments or insured parcel post.