How much has the Bank of England destroyed the Pound?

The real thing
As established by Sir Isaac Newton upon an already circulating coin, the British £1 pound gold coin, in the 18th, 19th, and early 20th centuries, was the Sovereign, so named for its portrayal of the reigning British monarch. This replaced the old British pound, which was a troy-pound of silver, or 240 silver pennies.

So here's the question:

The unreal thing
Since Britain came off the gold standard, how much has the modern ersatz pound been deliberately depreciated by the Bank of England against Sir Isaac's "Golden Cavalryman of St. George"?

Going by the spot gold value alone, one 'proper' Newtonian £1 coin is now worth approximately £180 fiat paper pounds (and rising). The Bank of England has therefore destroyed the pound by taking it down to 1/180th of its original value, in 80 years, or about 0.56% of its original value, gently stealing the other 99.44% of its value from you, over time, and handing it over to the British government to waste on statist fantasies and other garbage.

This process will continue ad infinitum (and probably at an ever-accelerating pace) until they take it down to zero value against gold (or silver, or fish, or loaves of bread); which is the fate of all fiat currencies.

But fear ye not, pilgrims, because not only has the British government ripped off your money, over your entire lifetime (unless you're a hundred and three), it is still trying to rip you off again, on the sale of these gold coins; because you can still buy freshly-minted sovereigns (which probably tells you everything you need to know about how much the British government really trusts its own fiat currency). And because all sovereigns are still legal tender, as well as paying no 'value-added' tax on their acquisition, which is nice, they are the only gold bullion coins which Brits can buy which do not attract capital gains tax. (Which, alas, puts a small premium on the little monkeys.)

So how much over spot is the British government charging you for these coins, throwing in a complimentary gift case worth about tuppence ha'penny?

Yes, folks, that's right. For a measly 22% markup, you can still pick up one of these coins from the British government's royal mint. Fantastic! Thank goodness for the blessedness of government.

Sir Isaac Newton, once Master of the Royal Mint at the Tower of London, must be fuming in his Westminster Abbey sarcophagus, as he gazes across the road to Parliament and HM Treasury to witness the destruction these fiat-money men have done to his golden legacy, and to the rest of the country. He used to hang counterfeiters by the wagon-load. I wonder what he would do to Gordon Brown, Alistair Darling, and Mervyn King, if he could arise from grave? As a man of deep mathematical principle, I'm sure he would think of something interesting.

(You can still pick up mixed collections of older sovereigns for about 3% over spot, if you search about.)