Sorry for my absence - been watching the tribute to Henry Allingham. God, what we could have learned from people like him. Clouding up here noticeably, and even more humid-feeling than before.
Sorry, didn't phrase my question properly - I meant to ask what mechanism would have caused it. I take it to be a shortwave trough rather than a longwave one, as my understanding is that depressions and associated fronts occur around longwave troughs and jetstreaks - so, given that, I assume it is possible to have a shortwave trough ahead of a cold front? It's on in the living room - now going for my dinner so I'll see the Look East forecast.
Also, earlier, I guessed, incorrectly, that the line of storms running up from Dorset were due to a trough, when they were, in fact, a result of the slowly-advancing cold front. So, where has this trough appeared from???
I was like that until VERY recently - now, you could fit my knowledge on a pocket notepad sheet!
If you look at your photos, near enough the centre of the storm cloud, there's a bit that looks like a funnel (or a nipple ) hanging down - this suggests rotation in the storm and can spawn a tornado.