Two options for the failed illegals rather than letting them disappear into the community.
Maybe a separate prison where they have to work for their keep and for this country but without any of the luxuries like tv or phones.
Use one of the seized boats and drop them just outside of French water with the tiller fixed so it goes forward to France, that way they are being returned from whence they came.
Trouble is the bleeding hearts would not approve of either option or any other solution however sensible unlike mine.