Ermm, you kind of have to come here first to find a job.
Also if we only let highly-skilled immigrants to come here, surely there would be too many high skilled people here, plus then people would complain about all of the high-skilled jobs being taken by immigrants and thus British citizens would have difficulties in finding those?
Another point would be that you do need those who are low-skilled, as there are many jobs that say English people wouldn't do, simply because probably they want to aspire a bit higher. I am not saying that no English people would take factory jobs etc, but it would be certainly harder to find them, as immigrants often will take those jobs.
Also, a lot of those immigrants have degrees from their country, which unfortunately are not acknowledged here most of the time, and therefore end up having a low or semi-skilled manual job. I don't think me or you would be happy doing that, knowing that we have degrees.
I am not defending immigrants, and of course there are some that just come and sponge off the state, which is a simply our of order, however I do believe that having them here does benefit the economy. The main reason for most immigrants come here for a job, not to get benefits.