Banks are being put on notice that negative interest rates Might happen. They will need to ensure that their computers and accounting systems can cope, that they have procedures in place to notify customers of the "charges" and so on.
But, think it through......
Bank of England sets a negative rate. High Street banks apply it. Loads of customers show up, demanding to withdraw their savings in cash. Banks run out of cash. There is a "run" on the banks and consumer confidence is wrecked.
My thought, for what it's worth, is that charges will only actually apy to business accounts holding cash, who can "afford" to recoup losses from their customers, or absorb them as a "business cost". If you try to charge personal account holders, there must surely be some minimum limit below which it is not economical for the bank to charge, and customers will just farm out their money to loads of different financial institutions, to keep each one below the charging threshold.
Of course, interest rates might actually go up in the meantime!