One way to find the windshear layer is look for a loop in the hodograph
The Camborne ascent shows one at ~600 hPa which Martin has already identified. Back trajectory analysis (NOAA HYSPLIT model) shows that the mid-level airmass originates from over Canada whereaes the higher airmass comes from a lower latitude source associated with the weak mid-latitude jet.
It's notable that the boundary layer air in the west also originates over Canada as well and airmass sources throughout the depth of the troposphere are now westerly in origin. The OOZ 300 hPa analysis has an arctic jet streak over the Labrador Sea which is presumably the immediate trigger for the regime change over the whole UK later this week.