Well I assume that you are still alive, so you should have ample time before Rigormortis sets in

Well if you can't maintain an address in the UK, you just notify the bank of your departure, and you just won't be allowed to top up the accounts. You will still receive your income from your savings etc.

Depending on why you want to maintain onshore accounts, I'm sure there are some alternatives. If you just want to take advantage of higher interest rates for savings in the UK, you will have to somehow maintain an address there, there is no other way, unless the bank allows non-residents to open the accounts.

Some of the nsandi.com products are open to non-residents.