Instability will build over The Netherlands, Belgium and NE France on Monday afternoon. A shortwave is expected to lead to destabilisation here on Monday evening, and with the approach of the Iberian upper trough merging with the Atlantic upper low ultimately backing the flow across central and southern Britain, any thunderstorms that do develop over the nearby Continent associated with moisture plume will begin to drift west or northwestwards across the North Sea. 

At this stage there is some spread as to the exact track of these thunderstorms - general consensus would put the risk higher over East Anglia and perhaps NE Kent, but some model guidance offers a more southerly option, even as far south as Sussex (and a minority of models keep any deep convection offshore to the NE of East Anglia). In either case, it is questionable as to how much lightning will actually occur over land (assuming deep convection does move inland) given the gradual reduction of instability through the evening/night. The risk (albeit lowering) will continue to migrate NW-wards along eastern coasts of England through the early hours of Tuesday.

For now, a low-end SLGT has been issued to cover the threat of weakening thunderstorms arriving late Monday evening / early hours of Tuesday over East Anglia / Kent, but it is certainly possible this SLGT area may need to be altered or perhaps even removed, depending on trends during Monday.