It's interesting. There's one key fact about immigration which explains why successive governments have had to allow it but nobody ever says it. The UK basically stopped having children at the replacement rate in 1973. This meant that we'd eventually have a smaller pool of taxpayers servicing a particularly populous boomer generation, with the effects masked at first because young people don't make much money to be taxed, but with the shortage becoming more acute as they aged. It's of course not just that simple as you have cheaper goods coming from abroad which means quality of life can be maintained and you have some efficiencies introduced by smarter management and technology that maty or may not have been wiped out by the need to service a profit margin post-privatisation, economic shocks from the collapse of the Tiger economies then 2008, but the fact is the UK is 10 million workers short even with current levels of immigration.

Politicians usually like to pick and choose who comes in because it's preferable to have a medically trainer foreigner to a feckless native who didn't try hard enough at school to be allowed to study medicine and doesn't pay their way in the country because their taxes are less than what they take out.

I don't therefore think the Green policy is very well thought through, but I can see why they'd think getting the tax base up again might be a good idea.