@Wil****
it's not that it's going to derail anything but if you can get 20k and the account is acceptable to immigration, why would you not take it? There's very specific rules as to when the money has to be there with the full balance needed for almost half the year.
Sounds easier and less stress to me to have a totally separate account for the 800k, earning interest, which you just leave alone.
Rather than moving money around and hoping you don't forget something or having an unexpected charge or direct debit bringing you down accidentally to 799,900 for 1 day (and yes they will reject for that).