Unless the tree is specifically excluded from the sale, it may not be removed as it belongs to the property.
It all depends on how long it has been there and how well established it is. Certainly it would need a large rootball with as many roots as possible to stand any chance.

Personally would not attempt... As Owdb states unless specifically excluded this is part of the sale but the main problem is trying to move a tree of 6 foot the roots will have spread to about the same diameter.

The tree has a long tap root that anchors it in the ground and the longer it has been there the more damage you will do trying to move it. 5-6ft makes me think it is around 15-20 years old and 20 years in the ground makes me think that it will do more damage than good.

On a practical side, they are damned spiky and you will need a couple of strong men to dig a trench cut a large and deep root ball and then move it.

I would also say to leave it when she moves.

I'd forgotten about the tap root Kildermorie!

I would buy a young one.

Thank you very much for all your replies - much appreciated. It was planted 3 years ago, we got it from a nursery, it's probably grown a foot or so since we planted it. I did promise to buy her another for her new garden when the time comes - I was rather expecting the answers I got! The spikes are indeed....spikey!