If mass immigration opens our job market to unfair competition, why will anyone here train as a plumber or learn any other important trade, if at the end of it they will be earning the same as a shelf stacker in Tesco?
It is why we don't allow unfettered competition with regards to import and export of goods, because exposing Western companies to the full force of competition from poor countries would decimate our industries - just as has happened with much of our manufacturing industries already.
I expect only when those supporting mass economic immigration into the UK find THEIR job priced down to the level of minimum wage due to massive competition from immigrant workers, will they pause and ask whether it is really benefitting anybody other than business and the immigrant themselves, not the country.