There is some doubt about the final results if and when all this is found. So far it seems to be a story of how detectors see something large underground and maybe a small camera was sent down and found something. This is not the same as a real intact Spitfire sitting on the surface, but it may be in the end.

20 ft 0r 30 ft of earth is very heavy, it is possible that the weight might crush a normal shipping container.
Some have remarked on another site, (warbirdinformation) that the "magnesium rivets" would have turned to dust. And I have heard this story from some looking to sell other planes, especially in the U S. I don't think so. The rivets are an alloy with magnesium to add strenght at light weight. They don't spontaneously disingtergrate at a certain time, like a new car which may be pretty trouble free until the month after the warranty expires.
If the rivets are kept painted and thus moisture free there may last decades. There are Spit MK I from before 1940 that are in museums and they did not turn into a pile of dust. If the rivets were painted and then sprayed with a protective grease or oil, and not subject to flooding, the plane could be in excellent shape.