Boon offers a virtual prepaid card that can be instantly added to your Apple Pay wallet and then topped up by Faster Payments (free) or 3D-Secure-supported card for a fee (but doesn’t support 3DS so that’s irrelevant anyway).
Their basic offering offers topping up to 200£/month, and there’s a “Plus” offering apparently, I am not sure about which fees (if any) apply. According to their pricing page there are monthly fees however the first 12 months are free (but I’m not sure whether this corresponds to the basic or Plus offer, or both). In any case 12 months should be enough for to support Apple Pay on their own!
I’m going to give this a test drive I’ll load it up to the max top-up level and go on a shopping spree. Will report back.