How do you prove sources of income?

P45? P60? S/E Accounts? Otherwise get a good lawyer/accountant when the tax man comes after you

I've never been asked ...

Good job really , as all my income comes from armed robberies

It can actually be quite difficult.
My earnings are sporadic and unpredictable, because I'm freelance - falling back on savings in quiet times.
I also have no/low credit score because I don't take out loans etc.
All of which means when my kids want me to be guarantor for flat rentals etc - which I'm well able to cover - I have to go through all sorts of hoops to pass the credit tests.

Get a free credit card

Then use it to buy stuff on Amazon occasionally

Then pay the card bill straight away

It will improve you credit score greatly

As I expected, the original post appears to have been a setup for spam.

Well spotted DB, and I've reported it.