Applied online
Got an email for an interview about a week to two weeks later. Got to select a day and time appropriate for me.
The interview was in a hotel near the store, as they were doing a large recruitment drive. Got given a name tag and told to stay in a room and watch a video about Primark. Then we had a presentation that pretty much answered every question you can think of.
One on one interview was pretty casual, if you're confident in yourself and answers, you should be fine. I got given question that were tailored to having experience in retail so they were a bit challenging but if you genuinely have experience in retail, should be okay.