I would also point out that Paris is the capital city. London pretty much already has the 15-minute city ideal. The tube network is world-leading in its technology and just how dense the network is.
I always reserve around 1h to go anywhere by tube. For places closer to me, 30-40min will do. Between walking to and from the stations, plus train wait times and walking between platforms for a change, the 15min idea is dead on arrival. Not to mention that this hour is filled with air, noise and visual pollution.
That said, I do have a small Saino’s about 10min from me, and a big Tesco 15-20min depending on how tired I am. If that counts as a 15min city, then you’re spot on.
For sure, it’s not perfect. But compared to the rest of the UK, you lot are pretty well off – generally there’s a pub and cafe within a short distance, and free access to most of the green spaces for exercise. Plus you’ve got plenty of buses.
It’s not perfect, but it’s pretty significantly better than my home in Bristol.
