This is just not on. And I doubt it will ever be implemented.
Yes, by all means ,check how much savings (grandly known as Capital to the DWP) benefits claimants have got - but do it by asking the claimant to provide statements. I think this happens occasionally already.
In the late 1990s some of my elderly CAB clients on Income Support were dismayed to learn that they could not give money to their DC like any ordinary person, as this would count as Deprivation of Capital. (These were people with a few thousand pounds, not change from downsizing or anything, all they had left at the end of their working lives.) It was their choice whether or not to do so, but if they did give money to their DC, their benefit would continue to be calculated as if they still had it.
HMRC and the police can ask banks for information about bank accounts. Rightly so - they are after actual criminals (even if some of whom go by the name of tax-dodgers
)
No reason to go after people with illnesses, disabilities, or other reasons for finding themselves living on a pittance through no fault of their own
And yes there is a black economy. The country would not run without it. It's at every level. Celeb freebies, a pair of grouse you shot at the weekend, all income that nobody taxes.