Know quite a few people that had an awful strain of flu December/ early January Time.
With the amount of travel between the Uk and Italy, China etc it’s likely it’s been here a while but on the other hand loads of people get bad flu every year.
The massive increase over the last few weeks does seem to coincide with the Half Term Holidays, people going Skiing in Italy etc. Expecially given it seems to be most prominent in London and the Home counties in particular