Hi, I am not to sure myself, looking at the BBC forecast. But the advice I would give is to watch the weather radar (live, updated every 15 mins or 5 mins if you pay) http://www.netweather.tv/index.cgi?action=radar;sess= .
The more intense bursts of rain would be more prone to Thunder/Lightning than less heavy bits. Also I would go on lightning maps (http://www.lightningmaps.org/realtime ), which is a good realtime Lightning detector. Although it doesn't record every strike, it is a good indication of where the storms are going.
Both those are more accurate than a BBC forecast.