Ef,
I would imagine that the LCY-ANR route can only work by offering a decent frequency - go down to 2-3 / day and people will switch to ES+conx. SEN is too far from Central London to attract a route like this that relies as Steviec9 says on city airports at both ends.
Steviec9 - I stand corrected on the free transfer. But my way of thinking is always train first - haven't driven now for nearly a year but that's another story! So yes, plane to car @ ANR is indeed very easy.
I agree it would seem like madness to run ES to AMS & not stop @ ANR - presumably not RTM either? My initial thinking was this might be to do with ppt control, but
afaik Eurostar takes domestic pax between Paris & Calais, so why not take pax between AMS & LIL?
Going back to SEN, I would still say that in the region 'reachable' by ES & conx (let's say 4hrs), AMS is the most likely destination, but DUS or CGN might be possible too?