Not rude here in the UK, or anywhere in the western world so far as I'm aware.

It is polite, though, to tell your host at an early enough opportunity that they haven't already bought the ingredients and cooked the meal!

Really you don't have to ask this so much as tell them - it would be inconsiderate of your host not to take into account your medical and religious dietary needs.

It's not at all unusual for people here in London to have particular dietary needs - eg halal, kosher, vegetarian, vegan, non-dairy, non-gluten, etc etc. If I'm organising the food for a social or work event then I'd normally ask people about their dietary requirements beforehand.

Not rude here in the UK, or anywhere in the western world so far as I'm aware.

It is polite, though, to tell your host at an early enough opportunity that they haven't already bought the ingredients and cooked the meal!

Really you don't have to just ask them what's in the meal so much as tell them your dietary requirements - it would be inconsiderate of your host not to take into account your medical and religious dietary needs and cook something suitable.

It's not at all unusual for people here in London to have particular dietary needs - eg halal, kosher, vegetarian, vegan, non-dairy, non-gluten, etc etc. If I'm organising the food for a social or work event then I'd normally ask people about their dietary requirements beforehand.