They're only in control up to a point. They can make new laws, if they can get them through parliament, but otherwise they're stuck with the current laws and/or the ECHR, under whose jurisdiction we are still, despite leaving the EU in 2020. And boy do these foreign criminals take advantage of that, together with Legal Aid, that most Britons can only dream of, and barristers to argue that the child they fathered with some random woman, who they neither see nor provide for gives them 'a right to family life'.
Either that, or bleeding heart lefties block them from being deported, if the British authorities ever get to that stage. Remember the Jamaican rapist who was in the process of being loaded onto a plane when do-gooding activists prevented it from leaving and he then went out and raped another woman? Yep, that's good old Britain for you! We never fail to be a soft touch and to give advantages and benefits to people who have absolutely no right to them, while penalising the law abiding citizens who work hard and pay their taxes.