20 days holiday? In the UK? That sounds like USA holiday time to me, mate. And if you're starting a new job in the US, that would be positively generous, I'm sorry to say.
(There's an additional provision that's a bit strange: the employee has to be able to use at least 24 of those vacation days in May-September. I think this law is a remnant from the agricultural society of the mid-20th century, when people in the cities actually needed those vacation days specifically in the Summer so that they could return to the countryside to help their families in farm work.)