The background of the incident should be that Britain intends to hold a referendum on whether to leave the EU at the beginning of this year, right?

à la carte means to order from the menu in French

French Foreign Minister Laurent Fabius said this sentence in the following context:

"We want the British to be able to bring all their positive characteristics to Europe. ... but you can't do Europe a la carte," he told French radio.

"Imagine Europe is a football club and you join, but once you're in it you can't say, Let's play rugby'."

Therefore, after analyzing the overall context, what he means should be: British people can't treat whether to stay in the EU as casually as ordering food in a restaurant.

In addition, French people's English is not so standard. The English they speak is full of French thinking, so it is enough to understand.