The best time of year to prune trees in the UK depends on the type of tree and the reason for pruning. For most trees, late winter or early spring (February to April) is the best time to prune. During this time, the tree is still dormant, which reduces the risk of damage or disease. Pruning during this time also allows the tree to begin healing before the growing season begins. It should be noted that, as mentioned, tree pruning varies greatly and usually takes place in autumn and winter. There is some good info on the Arb Association website. However, there are some exceptions to this rule. Trees that flower in early spring, such as magnolias and cherries, should be pruned immediately after flowering. This is because these trees set their flower buds in the autumn, and pruning too early can remove them. Other trees, such as oaks and walnuts, should be pruned in mid-summer to early autumn to reduce the risk of oak wilt and walnut blight. It is important to note that not all trees require pruning, and over-pruning can actually harm the tree. It is best to consult a professional arborist to determine the best pruning schedule for your trees based on their species, age, and condition.