Freshers week is a university wide event, not solely reserved for first years. Obviously in most places priority is given (rightly so) to first years, but it's definitely possible to attend as a second year.
When I returned for 2nd year, there was the lecture-free Freshers week and it applied to all years. The uni has an offical start date for when lectures began but I think enrolment day differed by one or two days depending on what year you're in. My 2nd year friends and I could still go to all the events that freshers went to, and we could hang around campus as much as they did until classes began.
The uni won't put on a 2nd year Freshers and you might not be able to get into (at least some) of the events as a 2nd year. However there's nothing to stop you going out in town the week before with your 2nd year housemates and students in other houses who you already know from 1st year and their housemates.