Unfortunatley the nature of the preview is that most of the effort is concentrated on merchant acceptance. If you take a look over here: Merchants & Banks Who Don't Yet Recognise the Monzo Current Account Sort Code / Debit Card (open Wiki) there is a list of merchants that flat out refuse to take Monzo’s details. Feel free to update if they refuse your sortcode.
You can also contact BACS to make a complaint that as a company is refusing to take Monzo’s sortcode, they are non-compliant with BACS regulations 
Hope that helps!