Hope this is true, might be quicker than if i order on contract with EE
This is the only reason I've not yet paid off my iPuP from last year. I can't be 100% sure they're using different stock, but based on the way the 8 works for existing iPuP members, it uses a different system of availability data, and passes around upgrade keys based on the phone you're upgrading from.
It could just be they're preventing you reserving 2 phones with that code I guess (there are translation strings for "You've already reserved an upgrade with that phone"), but realistically, based on what they did for Americans last year (phoning them up if you couldn't get the phone you wanted) - I'm guess is that they've got some dedicated stock for iPuP upgrades.
Those keys also make it hard to track stock levels on each system. Since Apple phoned me last year and asked me to stop publishing stock data, I've kinda avoided prodding too deep for the sake of them not banning me completely - but those keys would mean I'd be uniquely trackable, and it's not worth not getting an X
Also, here's a cool Easter egg for you that made me laugh. The reservation system has a translation sting calls "sosumi" which they're using to contain the text about capacity being taking up by the OS. The name "sosumi" relates to the alert sound they added into System OS 7 when they were going through the Apple Corp legal case.
Oh, and finally: I wouldn't be surprised if reserve-prime was only for iPhone 8 orders, and they used a different URL for X. Nowhere near 100% confident on this - but there are a few URLs which are returning access denied (over 404s) at the moment - which is a sign that maybe things will appear there.