You can see the point of imposing a time limit especially if it is a free car park as certain people take advantage, but these fines are excessive. Having said that when I lived in London the local Council car parks would charge £60 if you overstayed the time you had paid for, so its not just the private companies.
There is a Retail Park here in Norwich near to the football ground and within easy walking distance of the City centre and there is a constant battle with people being charged for overstaying. You could easily be in there for 2 hours as apart from the usual retail park outlets such as Boots, Argos, TKMax, Mothercare etc. these is also a Morrisons supermarket. The problem is there are 3 separate parking areas but the cameras are on the approach road so even if you move from one area to another to visit the various shops as far as the traffic cameras are concerned its all one event so its easy to overstay.