I really don't think it'll matter to be honest. I've been to each one of these launches and you have to remember that in order to have got a ticket you have to be reading Vogue, which of course means the people who are going are the fashion customers, not the usual H&M lot. With it being a preview event as well they don't restock as they would on the launch day, there's only a certain amount of stock allotted to the preview launch. For the Jimmy Choo party there were actually pieces missing, they only got put out for the actual on sale date.
The queue outside, even though people have tickets, generally begins around 4.30pm or so. If you're at the front then you should be fine. There's usually two womenswear sections too, one on the ground floor and one on the lower ground, so if you're taking someone with you it's best to get someone to get the pieces you want too, and then meet afterwards. That's how I've always done it, ha. This time though I actually I only want some shoes, so I'm not sweating it too much.