The O.P. is quite correct. Entering on a small boat without permission or permits is illegal and a criminal offence. So, yes, all of these people are criminals.
Some may have a good case for their actions and be genuine, desperate asylum-seekers - but it stretches the imagination to believe that they are seeking asylum from France. I know the language is tricky and the shopping hours also tricky and all sorts of things are difficult - but they aren't at risk of torture or sudden death. So, I would think that the genuine need to reach the UK is applicable to very few.
Another 'so' - logically we should be arresting, trying and imprisoning all these people - but there are too many of them.
As Hopkirk says, then what? We could fingerprint, electronically tag them and let them go. Ideally they would be moved to comfortable 'prison camps' and then sent back to where they came from unless that is a genuinely dangerous country. Other options ???
It'll need a big postcard.