As both UKIP and UK Hauliers called for the army to be deployed at Dover to screen out illegal migrants hiding in vehicles, there are several reasons why Cameron will not take this option;
1. OK, so they succeed in finding hundreds of migrants hidden in vehicles. What do they do with them? They can't be immediately sent back if they make an asylum claim - they need housing, feeding and care. And after a couple of weeks when the numbers are in the thousands and we need to build new detention centres, what then? No, far better, as far as Cameron is concerned, to allow those that make it into Dover to slip in quietly.
2. And no, the French would never stand for the British army being stationed in Calais to do the screening
3. The army are efficient enough not only to find the migrants hiding in freight wagons, but the hundreds a day more being carried in the boots of private cars for reward; for unemployed Asian lads from the Midlands, smuggling in migrants at £1,000 a pop has become quite an earner. Well, they're not going to Calais to stock up on booze, are they? Again, exposing the known problem to public view is less preferable to Cameron than keeping it quiet letting the illegals in.
4. Swarms of human rights lawyers / ambulance chasers will descend on Dover ready to blame the lads for every miscarriage, cut, bruise or hurt feeling and back it up with legal claims.
5. Having our lads stop the poor migrants coming in doesn't make as good TV as rescuing the sods from the sea etc. - best avoided altogether.
6. And having the army find large numbers who get through Calais is also bad news - news editors will back-calculate how many have already got in, and blame Cameron for a full five years' worth.
So I can confidently predict the outcome will be (a) to ask the French politely to do a bit more policing (b) to deploy the Territorial Voluntary Reserve National Barrier to join its regular cousin in Calais (c) er, that's it.