Currently there is a £2.50 fee for deliveries in London with Deliveroo. Even with extreme optimization a Deliveroo rider can't possibly make more than 3-4 "proper" deliveries in an hour (food gets cold fast! And when it gets returned deliveroo & the restaurant suffers) so maybe they can generate around £10/hour in revenue for Deliveroo. Then you need to take out costs for the driver, development, marketing, integration withe restaurants, etc.
So where is Deliveroo's margin on that? There isn't one, and there's no way they can pay delivery drivers as actual employees (which the law will eventually catch up with by the way), even at minimum wage you'd need to cover £15+/hour to include holiday pay, sick pay, insurance and other standard benefits.
MAYBE, delivery drones / robots will arrive before the funding runs out, but I doubt it very much.