We've known this for months...weak to moderate El Nino has been on the cards as well (possible Modoki), I'm sure Chris has other reasons but maybe it's the recent resurgence to that dreaded SW/NE dreaded jet showing up at precisely when winter starts has made him suggest this...
I'll be surprised if this winter is mild, with a raging jet and a constant conveyor belt of depressions similar to 2013/14 but it is the British Isles.