Too much hassle. Just leave the 800k in the bank and consider it dead money. No worrying about transfers or scrutiny by dim IOs. I once had to dip into it for 6 weeks to get bck to the UK when my mother died, but what a load of hassle explaining that later - show death certificates and all that bollocks.
The money must be in the account for at least 2 months before and 3 months after the visa starts, so there’s a 6 month window where you could use some but if you did, the balance must be at 800,000 three months before the retirement visa ‘anniversary’.
Best thing to do is just to leave the 800k in there untouched and forget all about it until you leave the country or die.