I think the distinction between Indian, Chinese and other common 'takeaway cuisines' and say French (aside from French not being a takeaway cuisine, normally only a restaurant one), is that there
is a generic Indian cuisine in the UK, whereas there is not a generic French cuisine.
French food, or German, or Spanish, is more peculiar to the particular restaurant, you wouldn't be able to predict exactly what dishes you'd be offered.
Indian food in the UK does vary, of course and there are some very fancy and now lots of regional Indian restaurants. But the 'mass Indian cuisine' we're all familiar with was established from the 1960s onwards, by people, mostly from Bangladesh, who were not cooks. They came here, spotted a niche in the (bland, post-war) food market and used a lot of generic pre-made sauces to achieve a standarised, nationwide cuisine.
That is the food we're usually talking about when we 'go for an Indian'. (Not always, there is increasing variation) but generally, its 'generic British-Indian curry house cuisine'.
Of course the proliferation and familiarity of Indian restaurants here is linked to Britain's colonial past. Just as in the Netherlands it's Indonesian and Malaysian restaurants.