Right, just found out some interesting info.
Went to my local apple store in South London. Apparently they were taking instore preorders but stopped that a week ago because demand was too high. (i assumed a reservation thing than an actual order)
Although the guy suggested I still come and queue up at 5pm, sounds like there'll already be quite a preorder queue anyway (with reservations).
Went to PcWorld across the road. They also expect delivery tomorrow, and will be on sale at 5pm Friday.
Interestingly they did take my name, number, and model I was interested in. She seemed to indicate that would reserve me one, but obviously not a guarantee. And she did say there'd been alot of interest but she only had 5 or 6 numbers written down)
So, seems like Pcworld might be the better option right now. So I might look at going to queue up at around lunchtime...'ish.