Mjolnir could never be part of the crown jewels. While Liz might well be worthy to lift the Hammer of Thor, in the intervening thousand years we've clearly had many monarchs (frex John, Richard III, & Bloody Mary) who couldn't lift it if they had Superman and the Hulk together helping them. Besides, the Hammer would eventually find itself coming to the queen whether she wanted it to or not. Uru will always find its center.
Besides, won't people have been asking over the centuries why even the most worthy British monarch (The Virgin Queen) wasn't using Mjolnir to blast the Spanish Armada into toothpicks before Sir Francis Drake and Lord Howard even saw the top sails of the first approaching enemy galleon?
EDIT: A fraudulent Mjolnir would have had to be introduced in the Dark Ages, so the validity of Thor's Hammer would have had almost religious overtones for England/Britain/Britannia.