Immigration doesn't help working classes get jobs. Not directly anyway. If you're a builder, and there are 3 other immigrant builders who are good at their job, quoting less than you, of course it does you no good.
What it does do, is stimulate the economy. Building is made easier, as there are more people available, its cheaper to build, and the quality improves. Which stimulates the economy, makes the UK wealthier, which in theory feeds back into communities, and creates even more building jobs. To be fair this has actually been the case with the UK, our economy is so focused on the housing market. People buying, doing up, selling, building homes etc. It wouldn't have been possible without migrant workers - many doing the unskilled building jobs and it stimulated the housing market leading to more jobs. Without migrant workers, we would need to train more people, which hasn't happened, and few Brits want the sh1t dirty jobs, so yes it has been essential to our economy.