Dogs Left in Hot Cars
First of all humans should not be leaving us dogs alone in cars at any time, but definitely not in hot weather.
What should you do if you see a dog distressed in a hot car?
This does depend on the level of distress.
It is not advisable to force entry to the vehicle yourself. Your first step should be to call the RSPCA on 0300 1234 999 to inform them of the details namely, the condition of the dog, the registration number and location of the car.
A dog warden service may also be able to help. They should dispatch an inspector/warden to deal with the situation if they can. they will call the police if it is necessary to break into the car.
If the matter is getting near life or death for the animal call the police directly and ask for an estimated time of arrival.
(I have only copied some of the important information)
The page I got this information from: