Anyone can dispose of £50 or even £5000. If your bank sees a £2k salary come in every month, they won’t be surprised that suddenly you have £50k in your account.
If you knock on their door and say, I’m a lowly administrator in a country in the bottom decile of human development index, can you look after my $100m, the should definitely dig deeper. In such case, yes you should actually document where the money is coming from.
It shouldn’t be such a high burden of proof. Money doesn’t spontaneously appear like particle-antiparticle pairs. Maybe you sold something, or were paid for some services. You should be able to demonstrate that with ease.
Remember, the bank, other things being equal, wants your custom. So they’ll apply the lowest bar possible to make sure the money isn’t stolen, lest the regulators slap them hard.
With CS the issue was, they basically didn’t even do the bare minimum with any degree of honesty and integrity.