Criminal damage to a memorial attracts a maximum sentence of 10 years’ imprisonment and a fine of £2,500
At the very least, it is a £20k fine for felling a tree under protection.
That's assuming no further impact on the site under various additional protection categories, which include up to 10 year prison sentences and uncapped fines. Given the significance and attention to this, could see it going quite far with it.
That's just the heritage and conservation side, not the forestry and ecological act, which has its own punishments.
Assuming anyone is proven to have done it, of course.