You would need to find a gateway that supports transactions for "high risk" transactions, which is typically what sales of CBD and CBD-related products would be classified as. There are some of these gateways that connect with Shopify that you could consider. I'd recommend checking out the following gateways:
I cannot guarantee that these gateways support CBD sales in the UK, but as gateways that allow for payments of more "high risk" products to be processed, I would suggest looking into using them first of all.
Victor | Shopify Social Care