It would have been politically embarrassing to have made a charge of theft as ownership would have had to be proved as would the intention to "permanently deprive."

The SNP only enjoyed some 0.7% popular support at the time and the Conservative and Unionists were enjoying a strong number of seats in Scottish constituencies. That isn't to say that they were not proud of being Scots even within a United Kingdom. Once the stone had been found at Arbroath Abbey the perpetrators obtained a folk hero status that crossed boundaries of political opinion.

A better charge would have been of criminal damage given that the stone was damaged as was part of Westminster Abbey - a "prank" that went badly wrong. But it would probably have appeared to be heavy handed treating what transpired to be an idealistic bunch of students this way and created resentment where none had existed.

And of course there are controversies as to whether the English ever had the original stone in the first place or even whether the one that was returned was the one that was stolen.

And Hamilton went on to follow a legal career so he most certainly would not want his student capers to be considered criminal. Interestingly he refused to attend the repatriation ceremonies it appears.

I found some reviews of the movie and was very taken with one which stated:

"A woeful slice of sentimental whimsy that makes Braveheart look like a documentary."