I think it maybe from thundery showers that are expected to develop in the wake of the front later today.
......... although now heavy echoes appearing leaving N.France - perhaps this is the source.
Totals starting to mount up now with the odd green pixel (32-40mm) around Guilford and south Lincolnshire.
It took a while to develop but it certainly has this afternoon south and sw of London and it was the frontal band that did it - and still is doing so - I should imagine over Brian's house right now!
That very cool air now seems to be over Cornwall. Between 11 & 12C all day so far. May end up a new June record low max (since 1992.) unless it warms up overnight. My lowest ever June max being 13.0C. Last Friday came close at 13.2C, and that was just before the observation time of 09:00 the following day, the same thing might happen again.