Read with mother.
https://www.sentencingcouncil.org.uk/offences/crown-court/item/criminal-damage-other-than-by-fire-value-exceeding-5000-racially-or-religiously-aggravated-criminal-damage/
Culpability - It's A or B, probably A. The level of planning is either way. No revenge and no risk to persons but definitely intention to seriously damage the tree. The wall damage being intentional or reckless will barely play. It's about the tree. The wall financial value was so low as to make it an add-on only. I don't think there's anything from C to nudge an A down to a B. Call it A (high culpability).
Harm is category 1.
A1:
Starting point - 1 year 6 months’ custody
Category range - 6 months – 4 years’ custody
Maximum: 10 years’ custody (basic offence)
Then start nudging it up for things like do they have a load of form, were they on bail for something else when they did this, was it because they hate foreign trees etc. Let's assume neutral for this since we don't know anything (or I don't anyway).
Then continue nudging it up for other listed aggravating factors. Three of the list of ten definitely play. Nudging the starting point up 30% gets us to 23 months so far.
Then nudge it down for bleeding heart liberal stuff. Are they sorry etc. They'll have a sob story of some kind but I can't see much washing. 5% off if they're lucky. 22 months.
Then some technical/procedural stuff like did they cough early or did they put the prosecution to the trouble of a full trial? Full trial so no credit.
Adjust for totality. Nothing complicated here. Very isolated offences. Maybe a bit extra for the wall. Maybe not.
So on the guidlines and with the information I know (not much - I don't read much news) 22-24 months. I'd say they court will want to give a particularly harsh sentence to give the public blood but that's why there are sentencing guidelines. 24 months is pretty much in the middle of the range. Easy for a judge to go with that so as to limit accusations of undue harshness/leniency. Let's not rule out an angry judge giving them a walloping one and saying, "**************** you. Appeal it on your own time. I'm off to lunch. You're off to prison." though.
There's probably some stuff about how much time you spend in an actual prison before being let out early or whatever. I don't know anything about that. If I had to guess, 24 month sentences, 12 months inside and then let them out once the point has been made. But I really am guessing there.
Sentencing and mitigation is a whole sub field of legal work btw, like stump grinding. There are briefs who specialise in being good losers.
General point: The courts punish offences against property disproportionately sternly compared to offences against the person. Someone up the page pointed out rapists can get comparatively derisory sentences. Yep.