- The Eurostar travells from Brussels-Midi to London St Pancras via the French station Lille Europe.
- The Eurostar has an unusual border-control arrangement, whereby passport control is done while boarding the train.
- France and Belgium are both in the Schengen Area, which means that people can pass freely between them. The UK is not.
The practical upshot of all this is that UK border control agents in Brussels check the passports of only some of the people getting on the train. Those who have tickets only as far as Lille are not checked by UK border control. It is then fairly easy for them to use standard fare-dodging tactics, stay on the train beyond the station where they should have alighted, and get through to Britain without going through any border control.
The simple and obvious solution would be to change Point 2 above, and do the border control in London, at least for the Brussels trains (this problem doesn’t apply to the Paris trains: they also pass through Lille Europe, and many of them stop there, but for pick-up only, not set-down).
That’s the simple and obvious solution. The solution the UK Border Agency actually tried was to attempt to profile “Lille loopholers” and interview them. The Belgian police soon put a stop to that: The UKBA has no legal authority to interview people travelling between Belgium and France. In at least one case, Belgian police actually threatened to arrest UKBA staff.