The point about temporary traffic lights is that they need road orders in place to make them legal, this can take days if not weeks to do and so many companies dont bother unless they are going to be there for a long period of time!
The key word in this is "SAFE" it may be legal to proceed but if you have an acccident once you've passed the red light then it obvioulsy wasn't safe and you'll be prosecuted, if a police officer sees you doing it then they have to use their judgement as to whether it was safe or not for you to go through the red light 9 times out of 10 they'll probably decide it wasn't and you'll get prosecuted! The chances of them charging you for driving through a red light is very slim and it would more likely be a case of dangerous drive/driving without due care and attention!