This is far from universally true, especially Asian takeaways typically hired their own driver, but the practice was also commonplace for many actual restaurants too
Many of the local restaurants will deliver
I think perhaps many commenters here are very young, so when I say restaurants did not deliver they cannot remember how it was 20 years ago. Order over the phone for delivery was limited to a few types of food places (mostly pizza and fast food made by the same establishments).
As an online platform Deliveroo was perhaps not the first (Just Eat is older, though not as focussed on delivery) but in the UK (and perhaps Europe) they really made that service explode. Note, though that I did mention Deliveroo and similar services in my original comment, not only Deliveroo. I think this is also linked with the rise of smartphone and apps, which, really exploded in 2007 with the first iPhone. The same goes with Uber.
Even takeaway in a restaurant was not really a thing and asking for a doggy-bag would only get you a sneer (and often still does).
When I was in Uni in both France and UK (up to early 2000s), if you wanted food delivered it was pizza, kebabs (in UK), or perhap Chinese.
This really changed over the last 10 years or so. Deliveroo was founded in 2013 and they are the ones who really made that explode.
Delivery for traditional restaurants has become a thing in the last 10 years or so when online services popped up.
It's strange to argue about this...
Before Deliveroo (and similar services) restaurants (in UK/Europe since you discuss London) did not offer delivery at all.
Handling the whole order (which these days means online/through an app) and delivery in-house is costly and restaurants use Deliveroo because it makes commercial sense.
Generally the issue is that, on the one hand I don't think that people are willing to pay much for delivery while, on the other hand, delivery is very low productivity and it's not viable to pay delivery drivers much for this.
As someone else commented 20-30% of an order is not crazy compared to the cost of serving on-premises. Moreover, Deliveroo can only charge this to restaurants if they have a deal with them.
Low productivity and cost pressures are also why "dark kitchens" are developing: They save on the cost of having a full-blown restaurant geared for on-premises service and they cut travel times for drivers (they can go to a single location and collect from multiple collocated kitchens).