dancingbear you've shown yourself to be rather unpleasant, but I will answer your points.
Last, no-one is say that all borders should be open. All people are doing is challenging the usual xenophobic tropes:
(i) they come here and take our jobs
(ii) they don't want to work, they just sponge off our benefits
(iii) the country is full up
So if you're not proposing open borders, then you're in agreement with limiting immigration. How would you limit it?
There's no evidence that immigrants rely on benefits any more than native populations. I haven't seen any mention of this.
As for the country being full up, well, there was a 10% or so increase in population in the noughties and that's overwhelmingly down to immigration. The UK now has amongst the highest population densities in Europe. Quite separately from immigration, the UK really needs to focus on developing other corners of the UK so as to redress the SE/rest of UK imbalance but it's not on to suggest that we just build all over open fields to accommodate immigration.
Does anyone care about declining biodiversity? I certainly do.
As for immigrants 'taking our jobs', the point that has actually been made is that a freeflowing tap of cheap Eastern European labour has impacted communities enormously. Someone made a point about the Lake District workforce being broadly supplanted by Eastern Europeans over the past years, and the response is 'oh well that's the rise of the super-rich and second home ownership driving out the locals', as if the current work force doesn't need to house itself? Of course they do, they just accept a lower standard of living i.e. they have driven down real wages.
Immigration on the main is good for the economy, but firstly, there are winners and losers and secondly, people are quite right to worry about the effect of a growing population on the environment.