Ancient myths, folklore etc. It is our 'national tree'. Loads of pubs, place names for villages, towns etc are named after it, songs about it (see vid), it is woven into our culture. It's called 'Royal Oak' because of its importance.
https://www.youtube.com/watch?v=4NXFCDgyanA (Royal Navy song)
The info in this link is good:
https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/english-oak/pic related is a pub in my village.