Just a question arising from the case of the Algerian street cleaners

We keep being told that, as a country, we need immigrants to do the work that the British are not trained to do. Really, is that really why Veolia Environment Services employ Algerian nationals as street cleaners? Are there really no British unemployed capable of cleaning the streets of London? Similarly are there really no British unemployed capable of issuing parking tickets in the City of Westminster, thus necessitating us importing 'skilled' labour from Africa? Why are we importing labour to do unskilled work whilst simultaneously paying Britons to be unemployed?