UKBA can check passports when people LEAVE London, but besides them being there for the pre-opening test runs that I attended, I've never seen the desks manned (but, the French ones are always there!). We only seem to do exit checks after something nasty happens it seems; reactive instead of proactive. Stupid, but that's how it is at airports most of the time too.
I've never been stopped or seen anyone from UKBA when returning to St Pancras (there's nowhere for them to be). They do occasionally have customs officers there, but it always seems like they're only working on tip-offs, so it does come over as quite an easy, safe, way to smuggle things in - that is unless the people scanning bags in France/Belgium do the tipping off if they spot something odd during the bag scans?
A comment on the DM does say that passports are checked at Brussels - by both Belgian and British officials (which is where UKBA are, in order to stop people getting into Britain in the first place where they could plead asylum).
If you were only going to Lille, I wonder if you would be able to bypass them? I've not been to Brussels for a long time, but in Paris I think every person has to be checked - assuming anyone would travel from Paris to Lille via Eurostar anyway, when I presume there are many normal trains that would be cheaper?