UK weather warnings have been issued by the Met Office, covering Thunderstorms over the next 6 days including today. It is difficult for us to summarise these, so we would recommend referring to their website for full details. Their home page is https://www.metoffice.gov.uk/.
The UK weather warnings are at https://www.metoffice.gov.uk/weather/warnings-and-advice/uk-warnings#?date=2020-08-12. Once there if you click on the yellow Thunderstorm label then you can see What to expect, What you should do, and the reason for the update.
There is a separate Met Office Heat-health watch which you can find at https://www.metoffice.gov.uk/public/weather/heat-health/?tab=heatHealth&season=normal. There is a Current watch level: Level 3 – Heatwave Action. There is extensive information there including links to a number of other useful resources.