The last serious thunderstorm i can remember vividly, was as long ago as 1978.
I was staying in St Dogmaels in Pembrokeshire, with friends, and we experienced several storms lasting nearly nine hours.
Such weather events are very rare in the Uk, a storm usually travels over very quickly, lasting no longer than the hour, or one sees the lightening from a great distance but it does not approach in your direction.
This multiple storm, seemed to last for ever, and my friend accurately gaged that it had lasted close on nine hours.
Since living in Dyffryn, i have not seen any localised lightening, heard thunder a few times, but no lightening.
The best way to avoid walking in such dangerous weather, is to carefully examine the weather reports for your chosen region.
Ive only been caught out in a storm once, in the Beacons back in the 1980s, huge black clouds approaching from the Merthyr area towards Cribyn.
By what i can remember, only large claps of thunder was heard, no lightening (thank goodness), but being right in the thick of it, there was nowhere to hide or shelter.
Thankfully their fairly rare events, and the best corse of action is to have an accurate weather report before you head out into the hills.
I like reading David Pledgley's interesting books on Mountain weather, he goes into great detail on the kind of weather one can expect with the cloud and pressure variations.
Well worth a read for the walker wanting to know a bit more about their environment.