He is a UK citizen and subject to HMRC rules. In the same way he was subject to UK law which is why he was made bankrupt in the UK ( for 5 years rather than the normal 1 due to his activities).
Is this the case? I’m a UK citizen, domiciled in Poland, with a NT HMRC tax code, being paid in UK but taxed in Poland, as l live in Poland for more than 193 days per year. If he’s based / domiciled in Monaco for more than 193 days he’s subject to their tax laws.