It’s certainly safe to say that it has been a very mild winter in the South East of England. So mild, in fact, that a sheltered Salix caprea where I work never lost all of its leaves (many remained green and attached, though I don’t know if they were actually photosynthesising much), and some Crataegus monogyna were coming into leaf in mid-January (the hedge line was south-facing and sheltered by a line of Quercus ilex to the north). Over the past week and a half, I have seen near enough all of the Prunus cerasifera ‘Pissardii’ come into flower, and the one I’m growing in the garden has now begun to break into leaf (I have attached some photos below I just took). Again south-facing, though it still strikes me as slightly earlier than normal. I somehow doubt that plants already coming into leaf will suffer as a result of this, assuming there is no late frost, of course. There’s also probably not much risk of the photosynthetic aparatus in deciduous broadleaved trees becoming ‘tired’, come leaf fall (it’s only a touch early).
As a partial aside, it does seem to be limited largely to Rosaceous species – always early to flower and leaf, this year they’re just that bit more eager.