It’s pretty much assumed that most major retailers, including the online ones, will begin taking Nintendo Switch pre-orders sometime after the Switch Presentation. In the case of Amazon UK, we know exactly when they will go up for reserve: 7:00 A.M. BST (6:00 A.M. GMT) on Friday the 13th. They mention:
Please be advised that Nintendo Switch will be available to pre-order from 07:00 BST Friday, 13 January, 2017. Due to limited stock we are only able to offer one unit per customer per address. We will be fulfilling customer orders in the order in which they were placed.
So, if that’s your retailer of choice, you’re going to want to go here and refresh as the time nears.