The Met Office maps suggest a load of scattered heavy showers heading in our general direction; their radar traces so far today are consistent with it. The aviation forecast gives a fair chance (P~=0.4) that we'll see passing cumulonimbus with thunder in the Cambridge area any time between now and the middle of the evening.

From the Lifted Index and CAPE maps on http://expert.weatheronline.co.uk/ I'd say we're looking at more serious thunder tomorrow across most of the south of the UK.

I used to set some store by uk.weather.com, but it seems to have gone off the boil recently.