IF someone were to win the £100m, where would you put it so that it's safe. You couldn't use a bank as only the first £35k is protected and that's an awful lot of different bank accounts you'd need to open. You'd also need a damn big matress to store it under.
If you had that kind of money you wouldn't be using a normal bank account and you'd be able to pay for expert advice on how best to save/invest it.