The idea here is simple. If there were unlimited cheap labour overseas, foreign companies would be able to produce very cheap goods which would undercut UK firms. Those firms would then either close down - which would raise unemployment and cut UK wages - or would force their workers to accept lower wages. Either way, some British workers would be "hit hard." This would happen so long as there is free trade, even without immigration. This is factor price equalization, which is a cornerstone of basic trade theory.
In pretending that British workers can be protected from cheap foreign labour by immigration controls alone, Ukip is therefore rejecting factor price equalization.
Now, to some extent it is right to do so. FPE doesn't hold precisely*. But there is some truth in it: it's no accident that the relative labour market conditions of unskilled workers in the west have deteriorated since the entry of China and India into the world economy.
Nevertheless, this poses two questions for Ukip. One is: if unlimited cheap foreign labour is kept out of Britain, what do you suppose it will do? Many of the answers involve threatening UK wages via trade rather than via immigration.
The second is: if you're rejecting basic economics in one big regard, why are you so keen to accept it in others? Just as it's too simplistic to say that FPE holds completely, so it's also too simplistic to say that immigrant labour greatly bids down domestic wages: immigrants are complements to British workers as well as substitutes, and even if wages are bid down, the consequent disinflationary pressures should allow the Bank of England to run a looser monetary policy thus providing a boost to the demand for labour.
Ukip, then, seems to be rather selective in its faith in basic economics. Call me Mr Cynic, but I suspect that such selectivity might not be entirely based in empirical evidence.
* In this context - yet again - there's a useful distinction between mechanisms and models. As a precise model of the world, FPE is inadequate. But as a mechanism/tendency it contains some truth.