In the most of the UK winter has been notable only by it's absence.
Yes, days are shorter, the sun is weaker and lower in the sky but the average temperatures have been much much higher. In December the South East averaged 8 degrees and London 10 degrees. The change from the average (so Radio 4 told me) is like Usain Bolt knocking a second off his 100m world record+. Apparently the North Pole is 30 degrees above average!
Trees need the regular rhythm of the seasons. If not the raised the temperatures (of air and soil) can trigger early spring responses (swelling of buds etc). If we then get frosts and colder weather these new growths can get hit very hard. The trees have to then spend energy repairing and rebudding/leafing. Losing precious energy. There are many other impacts of seasons not being seasonal.
In addition bugs that do not usually overwinter will not be killed off or decimated by the cold weather. They will be available in multiples of their former numbers to attack trees.
We need days of below freezing weather. To kill bugs and remind trees it is not spring. Please