(a) You can't assume that "UK People" ss you put it are qualified to carry out jobs which current employ migrant workers - this includes nurses, doctors, etc, and on the same note, whether employers would want to employ "UK people" who apply.
(b) If capitalists are denied "cheap" labour in this country, there is virtually nothing to stop them from moving production, and now even services overseas to exploit sweat labour.
A third issue is whether as tax payers we are prepared to subsidise employers who pay salaries on or just above national minimum wage, or offer very few hours employment.
As mentioned several times, migrant workers make a positive contribution to our economy through direct and indirect taxes.