It definitely is a scare tactic. I've already pointed out how the whole premise of the thread is ridiculous, but I'll do it again:
Why does the Jungle even exist in Calais?
Because the people there want to get into the UK illegally. They don't want to claim asylum in France (although they could do that), and the French are basically ignoring them.
If these people got into the UK, they would then claim asylum. They got to where they want to be! They aren't going to continue living near the port in a tent.
Cameron is trying to paint a picture of a huge tent village outside of Dover. "Have you seen that horrible camp in Calais? That'll be over here if you vote 'Leave'!". But this scenario would never happen. That's, like, the definition of a scare tactic.
The only reason for a Jungle on our side would be if there were asylum seekers in the UK trying to reach France, refusing to apply in the UK, and the British authorities were ignoring the problem.
There are very few of those for various reasons, mainly that the UK is rarely the first point of entry for asylum seekers (just due to geography), and usually their destination is the UK anyway.
