I think something like JustEat could be a quick and easy win for @Flux.
Since none of the takeaways/restaurants you can buy food from using JustEat are actually own by those companies, I imagine once someone orders using the JustEat app, a receipt is sent to the relevant takeaway/restaurant using some sort of API connection so they can prepare the food and have the delivery details.
So Flux would just have to build something on JustEat’s system which sends the same receipt details sent to the takeway/restaurant to them if a Monzo card is used.
The same would also be true for Deliveroo.
Of course it could be more complicated than I’m suggesting, but bringin a company like JustEat or Deliveroo on board would be an easy way of quickly expanding Flux across the country.