Why not? If I take something to a post office before 5pm and send first class or special delivery, it will get there the next day. Likewise, if I order something online, so long as I order before their shut off time, it will be with me by morning.
And not just Royal Mail; lots of companies offer next day delivery via courier.
Apple will use a courier like UPS and all couriers charge extra for Saturday delivery so the "standard" shipping is usally Monday-Friday. You may be able to choose express shipping but that usually entails extra cost on your part. I note that for the 5c they are offering free shipping to arrive on the 20th. We will just have to wait till nearer the time I think.