There is a system to pay for more than one day. Go to the helpful ladies on the information desk in the terminal and pay £2 per day and they will give you a ticket to put in your car specifying the date that it expires. You need to park in the long term parking area. If you are not certain about your date of return, perhaps paying a day or two extra is better than risking a clamping.
Surely an easier system is to pay on exit.
How can you know when you are able to return. You might know when you plan to return but incidents and weather can conspire to prevent that. It would be much easier to buy a ticket on entry and pay as you leave.
Or do they, like many other places, want the fines for over-staying?