"But earlier posts suggest that, quite correctly, many of the passengers would be for international connections."
Exactly so Phileas, which is why the through flight to AMS would be important. Who is seriously suggesting that many people would fly internally into STN or LTN and then go by train to LHR, say, for onward international connections so why single out SEN for that criticism?
Simpler to "nip across to NCL" is it?
Let's look at the facts. There are departures from NCL to AMS at 0600 and 0940, arriving AMS 0820 and 1205. The NCL website quotes a journey time from Carlisle to NCL by road of 75 minutes, so to catch the 0600 departure means setting out from home in the small hours.
From CAX you could depart at 0700 and arrive AMS at 1030 via SEN. That doesn't seem much of a contest to me.
BA from NCL to LHR offers greater felxibility of departure times, but many people would prefer to use AMS rather than LHR for their onward international connections I would suggest.
By all means let's discuss the pro and cons of the viability of scheduled services ex CAX but please stick to the facts and don't suggest the existing alternatives are entirely satisfactory when they clearly have drawbacks compared to potential routes from CAX.