It's gone a bit quiet, shall I re-cap?
This Vegetable is a living oak tree, which is not itself unusually old, nor is it one of the Great British Trees. It was royally planted (although a royal connection already existed) and is neither the Royal Oak in which King Charles mk.2 hid, Merlin's Oak or in Kent.

Being an ex-pat for nigh on sixty years this is becoming something of a guessing game to me. It's time to narrow the focus.

[Dujon] Yes, in jolly old England

[Raak] Yes, north of Watford.

That was otiose, reading above.

[Software] Yes, south of Hadrian's damp-course, and not in Yorkshire

[Phil] Yes, north of London (as implied by being north of Watford)

[Raak] Not the Midland Oak
I'm trying to think of a clue that isn't just giving away the answer...

[Phil] Not the Selly Oak

[Raak] Not Queen Vic

[Phil] She is a female royal

[Raak] Not Queen Anne
[GL] Yes, a British Royal
[Dujon] Yes, the oak you could visit now was planted by our current Head of State but I'm not sure how helpful that is, she must have attended many tree plantings in her time... Keep finding out who the original connection was - I reiterate, she is a female British royal who is neither Queens Victoria nor Anne.