Controlled immigration via a system that ensures a nation can increase the talent pool in areas of the economy/a certain industry is definitely a good thing. This should lead to improved economic growth & provide additional jobs/skills for the industry.
The issue is when immigration is uncontrolled as this can put pressure on local areas & services; it's not uncommon for those who migrate here to stick together (somewhat understandably) which exacerbates the issue meaning that certain parts of the country experience a substantial increase in population in a relatively short space of time. Large amounts of low skilled/unskilled immigration of labour can undercut British workers leading to a reduction of living standards, wage reduction/stagnation & general ill feeling towards the new arrivals. This is seen to be even worse should those who arrive here receive benefits, although the numbers that do this are smaller than certain newspapers would have you believe.
It's definitely fair to say that most people that come here do come here to work; the problem can be that there is already an abundance of other workers, both British & those who may have come via immigration previously, who are already qualified to do that job which can lead to what I mentioned above.
Issues can also occur if the immigrants come from a culture which is incompatible with UK laws/culture; personally I'd describe the UK is a quasi Christian nation on the path towards secularism & atheism. Those from other religions (certainly extreme forms) can cause friction the UK populous where views disagree. Therefore, the government does have a duty to British citizens to ensure those who cause future tension in the country are kept out. Those who come here from different faiths should be willing to assimilate to the point where they accept UK law is paramount over religion and I think the majority of them probably are. However, large numbers of arrivals from the same religious/cultural background can mean the it becomes difficult for them to assimilate even if they want to.
Summary - immigration is positive for the UK as long as it's controlled & the nation imports the skills needed to grow the economy; unskilled migrants should only be brought in if there is a shortage of workers available.
It is worth saying that the next 20-30 years could see a massive increase in automation in many industries which will very probably jobs away from people; migration may not necessarily benefit the nation as much as it has done in the past if this happens.