1. UK-regulated PSPs shall carry out credit transfer and direct debit transactions in accordance with the following requirements:

(a) they must use the payment account identifier specified in point (1)(a) of the Annex for the identification of payment accounts regardless of the location of the PSPs concerned;

(b) they must use the message formats specified in point (1)(b) of the Annex, when transmitting payment transactions to another PSP or via a retail payment system;

(c) they must ensure that PSUs use the payment account identifier specified in point (1)(a) of the Annex for the identification of payment accounts, whether the payer's PSP and the payee's PSP or the sole PSP in the payment transaction are located in the United Kingdom, or in the United Kingdom and another qualifying State;