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Plus of course there are vast amounts of that "market cap" that would be impossible to realise in any reasonable circumstances
£600m is nothing really. It's about 0.7% of the 24h trading volume of BTC. You could realise those coins over the course of a few weeks with very little movement on the price. In the last 24h 6% of the market cap of BTC was traded. More than likely you could arrange a direct deal with someone like MSTR or some other whale who wants to get into BTC.
Satoshi's coins are another matter completely, any movement on any of those early wallets would be picked up almost instantly by a lot of people who would take action based on that.
James Howells had 8,000 BTC. Satoshi has ~125 times that many (1,000,000).
Plus of course there are vast amounts of that "market cap" that would be impossible to realise in any reasonable circumstances - such as the coins controlled by the keys on the hard drive lost in the recycling centre; Satoshi's coins; other lost coins; etc.