Firstly, there is a small risk that an isolated shower or two could develop during the late afternoon/early evening over England, in response to diurnal heating and low-level convergence. However, profiles look rather dry with a mid-level warm nose which would likely limit the depth of any convection in either case. Otherwise, the main period of interest is through the evening and night hours. Forecast profiles suggest the potential for a few scattered surface-based thunderstorms that could develop over northern France during the late afternoon/early evening period. In either case, elevated thunderstorms are expected to erupt along the leading edge of the EML during Wednesday afternoon and evening, initially across N / NW France and the English Channel, tracking northeastwards across CS / SE England mid-late evening and then across the Home Counties and East Anglia during the remainder of the night. Effective shear (both speed and directional) is supportive of multicells and eventual upscale growth into a mesoscale convective system (MCS), but also may promote some rotation in stronger updrafts which will increase the risk somewhat of locally large hail. 

It is possible that thunderstorms could develop as far west as Dorset initially, and while individual cells may track to the NNE the focus for lightning activity could appear to shift more NE or ENE as the atmosphere becomes a little worked over / modified further west, and storms continue to develop in the most unstable air along the leading edge of the main continental Theta-W plume. There may be more than one main wave of thunderstorms too - the initial wave on the leading edge of the EML, and then as a low level jet strengthens through the overnight hours this could promote both better advection of Theta-E northwestwards and provide forced ascent for further heavy rain and embedded thunderstorms (these most likely on the eastern flank of the rain shield) across portions of central, southern and eastern England. However, this is highly uncertain with a large spread between model guidance at present, but could bring the risk of lightning further west than the initial wave, depending on how the event evolves through the night. 

PWAT in excess of 40mm is rather high for UK standards, and brings the threat of locally large rainfall totals - especially where back-building of cells occurs - and hence the risk of surface water flooding. However, it should be noted that the spatial extent of the highest rainfall accumulations may not necessarily be in the same areas with the greatest lightning activity (i.e. displaced to the west). Lightning will likely be quite frequent at times, especially in SE England. Meanwhile, inverted-V profiles suggest the potential for strong gusts of wind, potentially as high as 40-50mph locally, and as mentioned earlier there could be some locally large hail from the strongest cells (again most likely in SE England).