Expressflight,
Southend-on-Sea is not, and never will be, in London, neither will STN or LGW or LTN or Biggin Hill or Lydd or indeed any others, outside of greater London, that like to call themselves a London airport.
You talk about Air South West operating NQY via PLH, well I just happen to have an inside knowledge of the history of the NQY route and at one time, for a long time, there was a direct NQY-LHR service operated by British Midland, then Brymon, utilising an HP7 'dinosaur' aircraft.
When Brymon sold the HP7 to part finance the purchase of the DHC-7's the service then became operated by a DHC-7, in one direction but not both directions, via PLH, an HP7 couldn't have operated in/out PLH but a DHC-7 could, and there was an uproar amongst the NQY 'regulars'.
Had there been another airport, offering direct LHR services, from another airport within striking distance of NQY then Brymon would have lost these passengers business, and all for the sake of a 15 minute transit in PLH, in one direction only, that added some 30 minutes to their journey time.
But there is/was no alternative within striking distance of NQY, the next airport up the road was PLH, a 2,500ft runway, a 600ft decision height, a 1500m minimum RVR (despite the runway being less than 1500m in length) and with an appalling weather record with the only aid being an NDB and even then, to fly from PLH to LHR would have been via NQY in one direction.
Next up the road was EXT but Exeter only had services, sometimes, to LGW, next up the road BRS, no services to London and then they might as well drive or train it to London.
But had there been a 'NCL' within an hour (ish) of NQY, offering a variety of direct flights to major hub international airports, then Brymon would have needed to think again, as Air South West would, and compete by offering direct services.
So AMS is not to the north of SEN? A direct flight from CAX to AMS would route out over Ottringham and in, more or less, a straight line between the two, you suggest that from CAX it take a southerly route, around the London area one way systems etc. into SEN for a 30 (ish) minute turnround to then route north-east from SEN to AMS.
Now I would presume that it would be a maximum of a 50 (ish) seat aircraft out of Carlisle, passengers will invariably always opt for a jet rather than a prop and smaller airliners, per seat price, are more expensive to operate than larger airliners.
From NCL there are B737's and those larger Embraer things, however many seats they may have, so straight away, even on a direct CAX-AMS service, the costs per seat, to the operator, are going to be higher than from NCL. But then you're suggesting that they further inflate this cost, per seat, by adding flight time, associated maintenance costs, overflight charges, fuel costs, an additional landing and navigation fee in each direction etc. etc. etc. and you still belive this can effectively compete with a Dutch B737 out of NCL with all the connections, on that same carrier's network, whereas the NCL-AMS fare is incorporated, and reduced, in to an overall fare rather than two seperate fares?
You chose to previously cite the scenario of an early morning departure out of CAX/NCL but neglected to think that the pax need to come back again perhaps arriving in AMS mid afternoon. They could have a direct flight to NCL and then a 1 hour (ish) travel home or they could f@rt around travelling, at a higher fare, south west to SEN to then travel north west to CAX before perhaps a 30 minute travel home, I know which option I and many would take.
A round trip CAX-SEN-AMS-SEN-CAX would take circa 7 hours, out at 0700 in the morning, back at 1400 and if it takes 7 hours for a round trip an 'evening' service would need to depart CAX by no later than 1530 to be back in CAX around 2230.
No aircraft down time to operate other routes during the middle of the day, one aircraft solely utilised operating 2 rotations a day makes for very high utilisation costs.