By 'immigrants' do you mean people who are here legally?
1m at any one time can mean anything, it does not signify permanently being on benefits. It could be temporary unemployment, sickness, PIP, UC, pension credit, housing benefit.
A lot of those people could have lived here for many years and now consider themselves British, have brought up families who are British. They probably worked and paid into the system, they are as entitled to government help as any of us who were born here, some more so.
The likes of the Mail and, up to a point, the Torygraph, exist to create outrage with hyperbole. There's always more to their stories than meets the eye. There are approximately 37.5 million people of working age in the UK. Of roughly 9.9 million working-age people, 22.8 per cent of the working-age population, are receiving some form of benefits, according to the latest DWP figures, in addition to 13 million above state pension age (67 and over). That amounts to 14.5 per cent of the population as a whole.