Thursday is the day the heat will peak, but there will also be the threat of some big downpours and thunderstorms. Should any storms start to develop in eastern and central parts earlier in the day, temperatures may not reach record-breaking levels, but at this point, that looks unlikely. By mid-afternoon, temperatures are therefore set to widely climb into the thirties in central and eastern regions, with maxes of 38-39c likely in the east and southeast.
Further west, there is a risk of some showers and thunderstorms developing from the south fairly early in the day, but this isn't certain. By the afternoon or evening though, there are likely to be some storms and downpours affecting western regions. That threat then starts to extend further east during the evening and overnight. Some very lively storms and torrential downpours are possible with plenty of energy in the atmosphere for them to tap in to at this point.
(Straight from the Met Office)