If you say that's what happened that's good enough for me, svejk. There are a number of reasons for falling incomes and not all of them are to do with immigration.
However, that was not my point. You suggested that some "indigenous Britons" wont do certain work for "peanuts". My suggestion is that they might if they were not paid the same or sometimes more peanuts for doing nothing. It would concentrate their minds if, when there is work available (and there clearly is because we are constantly told that immigrants are snapping up all the jobs), they had no money, no food and no roof. It's called supply and demand. There is demand for unskilled labour, the British will not supply it at the rate offered so other people will. The "skewing" of the market has come about because, as you say, Britons will not work for what they deem peanuts and the State is saying to them "Well never mind, we'll give you some money instead."