Right, I work for CPW in a smallish shop in a market town. We are not expecting ANY iPhone 4's!!!!!! Also staff are not allowed to purchase so I went to the local o2 and orange shop. Orange are only getting 1 16gb and 1 32gb in stock and o2 wouldn't give exact number but they are getting few!
Considering these facts I would be getting to the network store very early! and if I had a pre-order with Phones4u I would be seriously worried as they are quite low down the food chain compared with the other stores.
Only the top 100 stores are likely to get a reasonable amount of stock (think 20-30!)
Also it's been confirmed that CPW will also only be doing upgrades for o2 not new connections.