bank wants to know the actual business address instead of PO box what do I enter

Registered address is the address in the business register, not your PO box

No, that is not automatically the same. The registered address is the address where the business is actually established, as registered with the Chamber of Commerce. A PO box is a mailing address: practical for mail, but not a registered address. If the bank asks for "the actual registered address", it means the address that is in the business register, not where the mail goes.

Fill in therefore the address as it appears on the extract from the Chamber of Commerce. If you have multiple establishments, this is usually the address of the main establishment — the bank normally asks for that address, not for each secondary address separately. If you are in doubt whether your situation differs from this, for example with a head office abroad and a Dutch establishment, then the extract itself is the most reliable source: what is stated there is the answer. If you want to clarify at once which address belongs to which establishment, that aligns with the question in which countries the enterprise has establishments — banks often ask that in the same form, and it is useful to obtain both answers from the same source.

Why the business register is the standard here

The Business Register Act 2007 requires every business to register a registered address as a fixed, formal particular of the business. That makes the registered address a verifiable fact, whereas a mailing address is a choice that you can change yourself and therefore does not count as an official identifier. Banks ask for this type of registered information in customer due diligence precisely because they are verifiable — comparable to the question for the legal name of the company, which for the same reason may never be a trading name or brand name. The extract from the Chamber of Commerce remains in both cases the designated source to verify the answer against.