As a trainee volunteer hedgerow surveyor, I was taught that it’s a tree if it’s over 5 metres and if it’s under, it’s a shrub. That’s one definition.
In 2009, Mr Justice Cranston gave a ruling in the High Court on what a tree is, a totally different definition:
https://www.landmarkchambers.co.uk/resources/cases/high-court-gives-important-guidance-on-what-is-a-tree/?fbclid=IwAR0NJNQbhZEQ01l2RK5_mxAxHonnAFiQPekONlZ2Ln7qOF3qPEFFgPpBX4U
However, HS2 Ltd and their contractors apparently have a third view, as an arboriculturist explains in the video, and in practical terms they can cut down what everyone else would call a tree!