@Night Troll
Yes, clearly the missing money would be noticed the following business day.
Of course, since switzerland is 1 hour ahead, a quick evening flight to Zurich gives you time for a good night sleep before you get up bright and breezy and withdraw the cash as the bank opens, a good hour before UK banks start trading.
By the time people come in the next day and actually realise what happened, you'de be long gone with the money. If you've transfered money round a few times to fill the gap where you originally took the money from, it will take a few hours to follow the money and work out that something has actually been stolen, rather than a mistake or legitimate transaction.
As for the sentence, we have sentencing laws in the UK, which quite amazingly, rather than rating the seriousness of the theft based on how you did it, base the sentence on how much you were trying to steal. So someone walking into a bank full of customer with a machine gun will get less time than someone who transfers £1m to his own account fraudulently.