No law has been passed making the Union Jack the national flag of the United Kingdom: it has become one through precedent. Its first recorded recognition as a national flag came in 1908, when it was stated in Parliament that “the Union Jack should be regarded as the National flag”.[3] A more categorical statement was made by Home Secretary Sir John Gilmour, in 1933 when he stated that “the Union Flag is the national flag and may properly be flown by any British subject on land.”[82]

en.wikipedia.org/wiki/Union_Jack

Just because it’s not official by your niche criteria doesn’t mean it isn’t official.

official adjective agreed to or arranged by people in positions of authority

dictionary.cambridge.org/dictionary/…/official