It's probably that government policy towards illegal immigration was that it was fine, just look at the the USA now Trump is trying enforce immigration laws that have existed all along (just not enforced properly) and the establishment is attacking him for it. The Democrats are even trying to get their supporters to fight against ICE (U.S. Immigration and Customs Enforcement). May doesn't have the balls to do anything that will likely provoke such a backlash.
Exactly this, the Uk is seen as a soft touch, any hint of sending any undesirables back to where they came from brings out hordes of protestors. I was watching 'can't pay we'll take it away' and the guys had to evict a Romanian woman and partner and six kids, they could not speak any English and had somehow managed to pay a rental deposit on a house and then immediately defaulted on paying any rent for several months. via an interpreter the woman was wanting to know why she was being evicted and where was she supposed to go now? back to where you ************* came from should be the answer, not to the local council as she was advised because having the kids would get her emergency housing at taxpayers expense.