There are a gratifying number of sensible, balanced responses in this thread so it obviously is possible to have a reasonable discussion, although of course as someone said earlier it helps to define what you are talking about - EU or non-EU, legal or illegal.
It also helps to have some sort of factual starting point. Given that anyone reading the popular press might be under the impression that every single migrant reaching the EU has only one ambition - to get to the UK - it's worth pointing out that the UK receives far fewer asylum applicants than most other major EU countries. Germany receives at least six times as many as the UK (over 200k in 2014, and heading for half a million this year - versus just over 30k in the UK last year) and France and Italy at least twice as many. Even Sweden and Hungary have more asylum applicants. Most asylum seekers head for countries where there are existing communities speaking their language. The availability of jobs helps, although plainly if you consider the destinations mentioned above, this isn't a primary consideration (it would be harder for an immigrant to find work in France or Hungary than in the UK).
Another issue that makes it hard to have a balanced discussion is the idea that you have to be pro or anti when in fact many people including myself hold a position somewhere in between. Immigration has many benefits but plainly uncontrolled migration is not a good thing, either for the receiving country or the migrants themselves (the camps around Calais being a good example of this).
Is the UK full? Bits of the south East sure, but if the govt could only adopt a more sensible regional development policy that spread out the benefits of economic growth a bit more evenly, this would be much less true. There are parts of the UK where there is very little pressure on housing (where my nephew lives, ex council houses have been offered for sale for a pound!) but of course the flip side of this is that there are not many jobs either.