A combination of cold air aloft,  surface convergence and 200-400 J/kg of CAPE will help to trigger widespread showers across the British Isles. Convergence boundaries near the coast of E/NE England will increase the risk of heavier showers, however the highest instability looks somewhat south of the convergence zone so the forecast remains isolated for lightning at this time. An upgrade to Slight will be considered for somewhere in eastern England on Monday morning.