Alex I think you are going to have difficulty proving the supermarket has done much wrong. By parking in their space without permission you are trespassing on their land. This is a civil matter, not a criminal one, so the police will not get involved. I would not say they had the "right" to block you in but she is sending you a strong message here to stop trespassing on their property! Think of it from their point of view.
There was some recent press in my local town about residents who blocked in clampers for four hours, here
http://driving.timesonline.co.uk/tol/life_and_ style/driving/article3392822.ece
I would imagine if the clampers could have called the police to get them out, they would have done so.