The Government's failure to prepare for this made jaws drop in Brussels
Even by the standards of impenetrable Eurospeak this one is pretty clear. Article 17(3) of the Treaty on European Union stipulates that the EU's policy-guiding Commission shall be independent and comprised of people known to be independent. "They shall neither seek nor take instructions from any Government or any other institution."
The principle is reinforced by the individual Commissioners taking an oath of office at the EU Court of Justice before they formally take on the job.
In the earlier days of the EU, this one was publicly seen to run on rails, with Commissioners speaking of "the region I know best" when referring to their home State.