OK, I'll have a bash...
A negatively tilted upper trough interacting with a jet streak left exit formed a low pressure area near the Straights of Dover, this was in a prime position to entrain high theta-w (warm & moist) air from Northern France & force it aloft. At the same time severe thunderstorms were initiating along the French coast near Le Havre, these caught the flow across the Channel & ran into the highly favourable environment over SE England. This caused a series of thunderstorms to initiate in the unstable air around & to the South of the Low centre.
That's how I see it anyway, apologies if I'm wrong!