One of our electricians carries the emergency phone every night of the year. You reach a person who can help, not an answering service taking a message.
Switch off the main breaker if there is smoke, burning, or water
Unplug anything that was running when the fault happened
Keep everyone away from the panel and any wet flooring
Have a torch handy and clear a path to the consumer unit
One thing not to do
Do not keep resetting a breaker that trips instantly. It is doing its job. Forcing it back on three or four times heats the fault, damages the breaker, and turns a repair into a rewire.