If you check your bank account details, they should include the IBAN and BIC numbers. I bank with NatWest and pay for my Infinity on a monthly basis also but I’d recommend you contact Barclays beforehand. When you queue up at the Passport Office you’ll be asked how you’re planning on paying and probably be asked to provide you’ve got your bank details with you. The IBAN tends to include your bank account number and a reference to your bank (mine has something like NWUK on it). Easiest thing to do is check a statement online as it should show on there.