The first move in date will be the Friday before Freshers' Week which runs from the 10th-14th of September, so Friday 7th. Most people turn up on the Friday (about 70%) with most of the rest coming over the weekend before the Monday (occupancy rates were over 90% last year by the start of FW). Assuming you want to just turn up before courses start, there's nothing to stop you picking your keys up as late as the 17th- but obviously your new flatmates will have been getting to know each other for ten days by that point.