The décor and the menu tell you right away that this is a full-on bistro, serving food that's now hard to find in London and less common than it used to be in Paris. The Sacre Coeur offers precisely the sort of food you'd expect from its appearance- rustic and traditional, no pretentions to fanciness or "gourmet" and it does it decently. Its a huge menu for a small place, which indicates that some of the cooking is almost certainly done well before it's ordered, and frankly I don't know that they would serve up a large portion of coq au vin in 20 minutes complete with chunky lardons without doing that.
The a la carte menu is large and pretty comprehensive. That and the wine list won't break the bank. Also they serve a two course lunch in the week for £8.95, including bistro staples such as whitebait , snails, moules frites, croques monsieur. Many cafes struggle to get a decent cooked breakfast out for that, and last week I paid £5.50 for a bacon sandwich on the coast. Just context. The place wasn't busy when we turned up, so not a real test of service- but it was friendly, efficient and brisk, just as you'd hope for. And it was French. Decent coffee too, though they've taken up the modern terminologies (espresso, Americano etc) rather than the simple traditional "café" which falls between the two.
We have a vast number of restaurants in London- and a huge variety of places to eat. But it hasn't all been upside. Many restaurants like Sacre Coeur have been pushed aside by a range of different concepts from Nando's to barbecued& pulled via Pizza Hut, all competing for the casual meal out budget. There's a parallel with the fate of Diners in the USA. I wish more like Sacre Coeur had survived and I wish them well.