Nothing, unless you took them apart and sold them off in bits.

You could go to one of those people you send gold to off of the telly. And then sell the gems on Ebay.

Phew bids could reach hundreds and eleventys.
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The Crown Jewels and all their component parts are instantly recognizable to any dealer or collector anywhere in the world. Because it would be so difficult to find someone willing to try to buy stolen property like that, you would get an incredibly minute percentage of their actual material worth.

There would never be an instance that the British would willingly sell the Crown Jewels. It is in their national character to melt the Crown Jewels down and throw them into the sea before they would ever allow them to pass into other hands. It would be a case of "over my cold dead body"….

You would probably be able to get less than a million dollars for them IF you could get your hands on them. Stealing them would be incredibly difficult, as those guards are not there for decoration, but are some of the most highly-trained guards in the world.

Then, the rest of the world would be so horrified that the jewels had left England that they would join the British in hunting you down.
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Forty three trillion pounds.
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And you won’t see a penny of it.