The profit that's made is (IFAIK) just treated like regular income, so it would be subject to standard taxable rates depending on which tax bracket you fall into.
I'm unsure how this would be affected by that person living in (and I assume earning a taxable wage) in another country.
While there's a good chance they might not get caught by HMRC, if they do catch up, there could be a substantial backdated tax bill to pay.
I'd seek professional advice!
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