It is estimated that a current of 1+ mA (0.001A) flowing across the body is enough to stop the heart and cause death. So High Tension school power supply units have to have high resistance connected in series with the output terminals, to limit the max. current to 1mA.
A uk mains (240v a.c.) shock is seldom fatal because human skin resistance is very high, and we are usually wearing shoes and socks of insulating material, so we just get an unpleasant stinging sensation and let go the live wire or whatever is giving us a shock. .. The sort of place where electricity is more dangerous is where people may be standing on wet ground, or holding on to a metal surface which is well earthed, so a bigger current can flow to earth.
In an extreme case, I knew an electrician who survived a 38,000 volt shock, although it did cause severe injuries.