hey there, first of all, good luck with the start of your PhD!
well at some German universities the heating and electricity is turned off for two weeks at christmas/new year in order to save some money. everything is closed and no-one can come in expect with special permits.
haven't heard anything similar in the UK yet. still: in general, if you are unfunded, I'd say you come and go as you wish. one of my first-year friends last year came back as late as 20th January! even if you are fully funded (check your funding contract for details!), unless there are some critical experiments going on at the time, no-one will expect you to be in before the new year!