Wagamama and Nando's are arguably the only two big casual dining chains that people actually enjoy eating at. All the others are pretty unpopular (and rightly so, the food is often terrible): Pizza Express, Frankie and Benny's, Pizza Hut, Chiquito...
Nando's social media and culture presence is impressive for a restaurant chain of its size.
Pizza Express is a known good. It's not the best, but it's guaranteed to be consistent everywhere. It does the job when you just want a "good enough" pizza with little fuss.
In contrast, searching for the best pizza carries a risk of getting something subpar and much worse than even Pizza Express.
It's not much different - Pizza Express isn't anywhere near as posh as it thinks it is, and I'd bet the average spend per customer is lower thanks to the constant Tesco Clubcard deals.
Ah, there's me confusing 'British restaurant chain' with 'restaurant chain in Britain'.
I'm sure the UK is by far their largest market though, and it has had an impact on 21st century UK popular culture far away and above any other restaurant.
Can't believe they've forgotten about Nando's!
Easily the most culturally significant restaurant chain in the UK, from grime music to the ubiquitous "cheeky nando's" phraseology