Wolfie, do many people use Eurostar for Brussells - Lille trips? Surely Thalys caters for this?
There are TGVs serving this route as well.
The last time I took the Eurostar from Brussels to St.Pancras UK Immigration at Brussels Midi asked to see my ticket and stamped it. Tickets were not checked on the train but on arrival at St.P all tickets (but no passports) were checked to see if they had these stamps on them.
As a thought experiment I worked out how someone entitled to free movement within the Schengen area but not to entry into the UK could evade UK immigration. Of course he (for lack of a gender-neutral pronoun) would need an accomplice who was entitled to entry into the UK.
(i) potential immigration control evader (henceforth PICE) buys a ticket from Brussels to Lille and boards train at Brussels
(ii) accomplice buys
two tickets from Brussels to London and has one of them stamped by UK immigration while boarding
(iii) once on the train accomplice hands over stamped ticket to PICE
NB: I am leaving Ashford and Ebbsfleet out of the equation.
(iv) at St.Pancras PICE leaves the train ahead of accomplice and exits after having shown his stamped ticket.
(v) accomplice exits and proffers unstamped ticket. As he is entitled to enter the UK he can't be apprehended as an illegal entrant and as he is in possession of a valid ticket he isn't a ticketless traveller either.
I'm sure someone tried this since June (that's when I last travelled by Eurostar) and that's why they introduced checks on the train itself. But PICE can still hide in the loo and take the stamped ticket from accomplice shortly before the train reaches St.Pancras.
I have worked out similar "thought experiments" relating to Stansed Airport where passengers arriving from the UK and Eire get mixed up with other passengers before immigration control but as this is a rail forum I'll let those be.