Heatstroke

Heatstroke can be caused by too much contact with the sun, being outside in very hot weather or exercising in hot weather. To avoid heatstroke, you should stay in the shade, drink plenty of water, try to avoid being outside in the middle of the day and wear light clothing. Heatstroke symptoms can include dizziness, nausea, cramps, a high temperature and other similar symptoms.
https://www.redcross.org.uk/first-aid/learn-first-aid/heatstroke

If you suspect that you or someone you know has heatstroke, you should try to move somewhere cool, remove clothes, drink water and cover the skin in cool water. If symptoms seem more serious or efforts to cool them/yourself down aren’t working, you should call 999.
https://www.nhs.uk/conditions/heat-exhaustion-heatstroke/