'Heat lows' turning into deep Atlantic lows
 

This seems to have been a repeating pattern this year: harmless
looking 'temporary thundery spell' heat lows coming up from the south
then deepening and dragging in cool, damp air from the Atlantic,
unexpectedly ending a settled spell (I'm more used to these thundery
lows just coming in, hanging around then drifting off again). Happened
on May 25th and June 13th, and the latest GFS (yes, a long time away I
know...) for July 11th suggests the same sort of thing happening again
following a week-long relatively settled spell for England and Wales.
What actually causes this to happen?

'Heat lows' turning into deep Atlantic lows