I'm in the Netherlands now, 'tjap tjoi' is pretty much the most cliche 'Chinese' (in quotes because it's all heavily Westernized) dish there is.
When I was living in the UK (Staines, south-west of London - but this was 10+ years ago) I remember recognizing it on menus, I hadn't had much Chinese at the time and this was one of the few I knew. But yeah I don't know how wide spread it is. Maybe the same decline in popularity the article talks about has happened everywhere.
I don't know where you're based, chop suey isnt really a Chinese restaurant staple. Growing up I remember a takeaway called Thai Garden advertised it, but I always associated with American films.
For takeaways the format was overwhelmingly headings with the meat, then items with different veg. Eg X with cashew nuts, X with water chestnuts, X with oyster sauce. Then an Egg fried rice section, then a Chow Mein section.
But that could be a regional thing within the UK.