Used to have German current account, left without closing it

You owe money to a bank in Germany (via a presumably unauthorised overdraft). The charges, both the regular banking 'relationship charges' and the penalty charges, will keep growing and if these continue to go unpaid, it will of course have consequences. You agreed to these terms when you opened the account and now, in the eyes of the law, you're stealing from them.

Contact the bank immediately and settle the accounts.

An example consequence, beyond debt collectors being involved, is if you want to lease an apartment in Germany in the future. Any credit check done will reflect badly on you.

Another facet is that many banks have higher non-resident fees for non-resident account holders. If you left Germany some time ago, the amount you rightfully owe may be even higher.

Finally, consider that after Brexit, there is a possibility that a visa will be required to enter Germany as a UK National. These unpaid debts may be considered in such an application.

Just call them. They will speak English on the phone - that should be an absolute standard for German bank.

Best case they will waive your open fees and retroactively close the account, and worst case they tell you how much you owe and close the account now.