There is a young Hawthorn in Todmorden that has had twigs in full green leaf since Jan 1st. But this early leafing may be caused by planting of European stock from different climate zones.
It is well recognised that most 'native' tree and hedge planting was done (and still is) using imported saplings. There was a good article in the Botanical Soc of Britain and Ireland magazine on 'Look-alikes', where many of the 'native' wildflower plantings were found to be imported. They were the same species but subtly different in size of plant, time of flowering etc.
Records of 1st flowering may be influenced by these 'Look-alikes'.