Found this about UK Royal Mail, maybe an option....
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The Royal Mail definition of valuables can include bank notes and coins (British or foreign), postal orders, cheques, dividend warrants that are uncrossed and payable to the bearer, coupons, vouchers, tickets or anything that can be exchanged for money, goods or services such as TV licence stamps or gift tokens, bearer securities including share warrants, bonds, relative coupons, scripts or subscription certificates, and jewellery, precious stones, metals or watches, (the cases of which are entirely or mainly composed of gold, silver, platinum or other precious metal).
Always send valuable items using one of our Special Delivery