As we don't have 100% employment of able bodied native adults then of course we have UK citizens who can do the work or be trained to do so. So no reason to try to develop a dependence on immigrant workers.
The side issue is the work should be paid sufficient to attract native potential employees in the work market, and the work conditions sufficient to do likewise. If pay and conditions are not such then the business isn't viable here and bypassing the national market to attract those poor enough to put up with less, isn't morally correct.
There are further side issues too, notably welfare payments, but the subject of a thread should be limited or risk going off track.