There is no reason why we cannot, Geezer, we just do not.
As part of some work that I do I once spent a day with the Dover Port Authority. I spoke to a number of immigration officials. Some of them were involved with searching lorries coming from France. They were instructed not to detain illegal immigrants who may be seeking asylum, but instead to direct them to Lunar House, Croydon. (They had pre-printed cards with, in various languages, directions how to get there). Of course, they were instructed to continue their proceeding against the haulage firms who unwittingly brought in the human cargo (usual fine �2,000 per person).
I also attended a seminar where a speaker from the Kent Immigration authorities explained the lengths his organisation went to in identifying and apprehending illegal workers in the county. What did they do with them? Gave them the same card with directions to Croydon. They are not instructed to detain them in either case as, apparently, they have done nothing to warrant such detention. Many of those apprehended have no genuine grounds for asylum, they are merely economic migrants.
The notion that ID cards will put an end to this is ludicrous. These people have no papers on arrival and many of them never gain any papers for the entire duration of their stay (which for many of them is the rest of their life).
Successive governments in the UK have seen fit to allow this situation to perpetuate. Despite what Vic would have us believe it is costing a fortune, but most of the costs are hidden. It is true that numbers have fallen, but we are still nowhere near robust enough in dealing with this issue.