Ireland and western Scotland...

CAPE values will approach 1,000 J/kg by mid-afternoon in response to daytime heating allowing surface based thunderstorms to develop across Ireland. A few thunderstorms are also likely to develop across western Scotland in response to weak convergence and topographic forcing, although instability will be less and forecast profiles are drier (compared with Ireland). 

Southwestern Britain...

Weak mid-level instability will be present across southern Britain, although midlevel profiles are very dry, so showers are expected to be very isolated, and not likely to produce lightning.