If you're the proprietor of a greasy spoon caff who struggles even with simple government forms and you unwittingly employ an illegal immigrant as your washer-up, you face prosecution and a fine of up to £10,000. It's no defence to claim you didn't understand the documents or the requirements for a valid working visa.
If you're a qualified barrister who understands professionally exactly what the requirements are, there's no justification whatever. And if you're the Attorney General as well, to employ an illegal is either an indicator of appalling incompetence that makes you unfit for office, or wilful disregard of the law. Either way, it demands a resignation.