I think you're mistaken Eric. Eurostar (+ immigration authorities) care a lot about making sure people with a Brussels to Lille ticket really get off in Lille (and don't stay on until the UK), with various special checks around this (dedicated checkin area, dedicated carriage, security staff etc). However, for a UK -> France+Belgium journey, you clear French immigration in London, and are then "in Schengen" for immigration purposes. There are no issues with you getting off in Lille (provided the train stops there!) on a Brussels ticket, no checks, no-one notices. Tickets from Lille to Belgium on the regular trains can be bought with cash for a few euros (special Trampoline tickets), there are no issues with ID checks or anything for an intra-schengen trip like that