If you're super eager to make sure you get your hands on a PS4 on release day you may need to pre-order it to make sure. Amazon have the PS4 up for pre-order at a whopping £599.99 which is ridiculously high.
If you're looking for the cheapest place to pre-order the PS4 you'll definitely want to look at Zavvi.
Zavvi have the PS4 up for pre-order at £399.99 which is cheaper than the launch price of the PS3 which went for £425. Hopefully Zavvi will stick to this price. They do guarantee to charge you the lowest price if the launch price were to be lower than £399.99.