If your account is a US account then UK customers aren’t required to go through SCA. If they’re getting declined then they should reach out to their bank to approve the payments.
If you want to see declined payments in your Seller Dashboard your Transaction Status report gives you information about any unprocessed transactions, including declined and cancelled transactions and other uncompleted transactions in which a customer’s card was swiped, inserted or manually entered, but not charged.
To view the Transaction Status report:
Visit Reporting > Reports tab on your online Square Dashboard.