We could probably do with a dedicated "immigration" thread. I'm not sure the "open borders" advocates would be many; I just do the history of how that used to work for centuries, and how immigrants' work ethic and innovation has helped the UK- all those jobs round here thanks to Beyer (as in Beyer Peacock), Renold, Ferranti, Marconi, Dreyfus (Clayton Aniline), Brunner and Mond (ICI).
I did recall Ken Livingstone saying London could not function without illegal immigrants, but when I searched for that, this is what I got:
http://www.bbc.co.uk/london/content/articles/2008/04/09/mayor_migrants_amnesty_feature.shtm)
(Johnson supports amnesty for illegal immgrants.)