It’s probably worth mentioning that this doesn’t just stop at legislation. A lot of things in the UK are the way they are, just because that’s the way they’ve always been.

What’s the official flag of the UK? It doesn’t have one. The Union Jack was a naval flag that became our defacto national flag. Before WW1, people could have lived their entire life without seeing a Union Jack.

What’s the official national anthem of the UK? It doesn’t have one. God save the King / Queen is our defacto national anthem. It was a song that gained popularity and people adopted it unofficially.

OK then. What’s the official language of the UK? You probably guessed - it doesn’t have one. English is only the defacto language of the UK. In fact, the only official language anywhere in the UK is Welsh, in Wales (obviously), where the vast majority of people speak English as their first language anyway.