One of the best ramen I’ve ever had is this: https://ramen.haus/united-kingdom/london/cha-mori-ramen-kana... Pre-Covid, this restaurant would regularly have queues down the street to get in. But it’s a small place. Basically the size of someone’s front room. Definitely worth a visit.
Kanada-Ya is a chain with at least 5 locations in London, and while they're fairly good, they're still somewhat focused on Deliveroo-type transactions. It remains quite different from a genuine ramen shop in Japan.
And it’s obviously going to be quite different from Japanese restaurants because London isn’t in Japan (surprise surprise).