Indeed, it's only recently that the UK government discovered a loophole in travel from Brussels by Eurostar. (Well actually they've known it for years but haven't bothered to do anything about it)

Someone with a right to enter the UK buys a ticket to the UK, the one who doesn't buys a ticket to Lille. The one with the ticket to Lille doesn't undergo immigration checks at Brussels (itr's like pre-clearance) whereas the other does. Once both are on the train they swap tickets and the illegal travels into London safe in the knowledge that there aren't any more checks.

http://www.telegraph.co.uk/news/ukne...-Eurostar.html