Originally Posted by
stut
[...]
Eurostar have also had an unofficial policy of letting you board at later stations (e.g. Ebbsfleet and Ashford) if you let them know in advance, but Lille may be a special case (given the well publicised 'Lille Loophole').
I think that was an official policy, but regardless it changed some time ago anyway.
The current policy is described in this
Eurostar FAQ:
Can I board the Eurostar from a different station?
Tickets from London St Pancras International are also valid from either Ebbsfleet or Ashford International stations, however please bear in mind that not all Eurostar trains are scheduled to stop at Ebbsfleet or Ashford.
If you wish to change stations and your train does stop at Ebbsfleet or Ashford just check in for your journey at the station of your choice.
If you have a ticket to travel from Ashford or Ebbsfleet International stations
Some tickets booked to depart from Ebbsfleet or Ashford International stations are restricted to those stations and cannot be used to depart from London St Pancras. This will be indicated on your booking
In terms of getting off a Eurostar train from London at Lille, whilst holding a ticket to Brussels - I can't see any problem with that whatsoever. You'll have already passed through the Schengen zone immigration check (operated by the French) in London St Pancras station before boarding the train.