No you won’t need to opt in, as long as you have online banking enabled for your bank account, then your bank will be ready to pay using Open Banking!
All major banks are required to provide Open Banking services to their customers.
You will retain all control over where and how your data is used. Therefore before making a payment, or sharing data from your bank, you’ll be asked to consent and approve the transaction.
All payments are approved this way, so you can be safe in the knowledge this is secured with the same measures as when you make a regular online bank payment.