Immigration is a double edged sword. Politicians work on the simplistic basis that you need people in work to support an ever increasing aging population, and as the UKs population isn't growing fast enough, then the growth needs to fuelled by immigration. Except that immigrants tend to have larger families, so that requires not only more housing, but more infrastructure, more schools, more hospitals, that then need more doctors and nurses. But because we haven't trained the doctors and nurses, we then need to import them from abroad, and so the cycle continues endlessly. Often, immigrants that come here for work, then bring their families with them, and then they are entitled to in work benefits that outstrip what they are putting into the economy.
We also have a lot of immigrants that are selling the Big Issue, and they are then entitled to the supposed in work benefits. Hence the reason that controlled immigration is needed.
And of course, then we get to the fact that Immigrants are needed to do the jobs that the home grown benefit scroungers won't do, because it's too much like hard work. So, sometime immigration is a good thing. sometimes it's a bad thing, and sometimes, it's a necessity.