Now if Britain is leaving, English could be considered as a neutral language so no country has an advantage.

It is also still the only language that is taught as a foreign language in close to all primary and secondary schools in the Union. It's the most spoken language in the Union. It's relatively easy to learn as a foreign language. It's also the de facto language of international trade, of science, of culture and of international diplomacy (sorry France).

Surely the case for English as a working language is now stronger. Previously it could have been seen as unfair to use one member state's language over another...

Which one can replace it? Either French or German will face obstacles.

Or, on the contrary, there is less reason against it (no more perceived advantage or "cultural capitulation" to the Brits)

Because English is one of the simplest languages to learn.

A better question is why not Spanish?

- Because Mandarin is the official language only of China (and Taiwan)?

- Because English already has a great momentum (with the number of speakers of English as a second language, pop culture and the Internet)?