http://www.bbc.co.uk/news/uk-16058860

I’d still say that coming off the train at St Pancras is still an intimidating part of the journey. The UK border is not a welcoming place, even if you happen to be a returning native. That said, I didn’t know about the “Lille Loophole” before today. The thing is, certainly at Brussels, there is your passport is checked both by Belgian police and a UK Border Checkpoint. After that, yes, no further checks. I am assuming that there is a similar arrangement at Lille (IIRC, there is at Paris).

Apart from Lille, the Eurostar tends not to stop, and you are sealed in until it disgorges you in Kent or London. So I’m not sure how someone can “sneak on” between stations. Nor can I see how someone gets onto the train unchecked by UKBA either. I’m not aware of anyone getting fast tracked past the border checks just because they are getting off at Lille.

I can see the Belgians' point - why are UK Border staff interrogating individuals in the sovereign territory of another country?

Did you not know this happened? It's easier for UK Border staff to stop people travelling to the UK than for them to be stopped at the border and turned back. I am led to believe the UK Border Agency has staff at most international airports around the globe.

This news came out a few weeks ago during one of the perennial political spats about immigration to the UK. Claims were made about stopping a load of foreigners and undesirables, implying it happens here in the UK, when it actually happened at the point of departure.

Sure, at airports and ports where the destination is the UK. I'm not sure it's reasonable to start interrogating passengers on a train who are travelling from Belgium to France. There may be an argument for questioning those who have passed Lille and thus are actually on their way to the UK.

For the Eurostar there is a special arrangement for Border Controls

In the UK you go through both UK and French Border controls (they are one behind the other)
In Paris, Lille and Brussels you go through both UK and French / Dutch Border Controls

HOWEVER you only go through UK border controls when in Brussels IF your ticket says UK. If you Ticket says Lille then you bypass UK Border Controls

However

Once on the train there is no check to see if you get off at Lille (except random ticket checks) so you can stay on the train to the UK and then as yu have in theory already done UK border controls there are no controls as St Pancras so you can just walk into the UK

Simple answer: Lille = get on only. You can travel to Lille from Brussels or Paris using the Thalys or TGV services. They are just as fast as Eurostar.

That would require the French to enforce it, which would require them to give a flying one about people trying to enter the UK illegally. History suggests they really don't.

Jon

The next question is this: what, if any, advantage is there by travelling into France and then into the UK rather than just "visiting" the UK and failing to leave?

You won't be scanned in, so you don't exist.

I imagine there's a long list on a computer somewhere of visitors who have overstayed their paperwork.

You may have got into France illegally so have no papers to get into the UK

I guess the answer is to have all Lille passengers on the same coach(es), and Eurostar staff (or police if necessary) ensure everyone in that coach gets off.