Hypothetical - Moving large sum of son's money from UK

[Seasider4374]

Whilst waiting for NS&I to send me my login details (and then being able to check my 11yo son's account status) I just got to wondering what IF he'd won a significant prize in the past 11 years?

Whatever his winnings (or lack of) I'd like to bring the money to Japan, and invest in a Rakuten account for him. And so what is the best way to do this?

I've imagined just closing his NS&I account, moving the money to my Lloyds a/c, then transfer via Wise to his Yucho account (and then to a Rakuten I'll set up).

Though I'm guessing I can just bypass the Lloyds a/c by giving NS&I the Yucho relevant SWIF/IBAN number?

If (say) he'd won £1m, what is the best way to send the money to negate his Japanese tax?

[Deep Blue]

If he has won a big prize, it is likely he will owe Japanese tax on it. Leaving it in the UK won't negate this, although in practice it seems unlikely the NTA could find out.