Surprisingly, there is no law declaring English the official language of the UK.
In fact, English is just the de facto official language of the UK, which means it is not legally sanctioned as official but spoken by the majority.
England and Scotland do not have any official languages. Northern Ireland’s official languages are English and Irish.
I’d known that about the US, but always assumed that England had English as the official language.
Mind you, the aristocracy and royalty spoke French for 200 years so I’m wondering whether English, or French come to that, was ever the official language.