All EU countries have different ways of expressing account numbers and formats for making domestic payments. IBANs provides a common style, (even though they look different) of account number format across the EU and EEA. It's easier for banks to identify if a number is wrong.
Strictly speaking, IBANs only apply to euro payments. However any payment to the EU or EEA can be converted to local currency. We strongly recommend the use of IBAN and BIC for all payments you make or receive from the EU and EEA including sterling payments.
No, you should continue to use sort codes and account numbers for these payments.
The paying bank may decline to make the payment, which could affect your cash flow. If the paying bank agrees to make the payment, it may levy extra charges for making the payment.
Yes. You can quote the IBAN as your account number and the BIC as your bank details in the same way that you present the information for the EU and EEA. The information is sufficient for the payment to reach your account at NatWest International.