The UKGC are going to kill the industry with this OTT BS. Asking a player to submit a signed declaration is one thing, but asking them to supply things like wills, estate valuations, letters from a rival casino confirming a win, is going to be seen as a deliberate attempt to avoid paying out, regardless of whether it has come from the casino or the UKGC.
The fact that these requests only seem to come when a player has a big win also adds to the BS because the answer to this source of wealth question is "I just won it from your casino".
It would be far better to impose this duty upon the banks who operate the deposit and withdrawal wallets. Only they can see the bigger picture of whether the player is simply recirculating their funds or routinely injecting fresh funds from sources that seem inconsistent with their status, such as regular cash deposits or fund transfers from hard to verify sources.
In any case, however it is done it has to come with a duty to provide players with the documents that other casinos will be asking for under these new regs. So, if a "certificate of winnings" is going to be a required document, casinos should have a duty to supply one to a player so that it's available to show to another casino.
The observation that high rollers seem to be able to avoid these checks for business reasons such as not upsetting them indicates that this is not simply something that has been imposed by lawmakers without any flexibility being allowed, OR these other casinos are flouting the law in order to placate their key clients.
What seems to have escaped the UKGC's attention is that prior to regulation dodgy casinos have often used documentation as a means to avoid paying winnings after the fact, and players had expected that regulation would curb some of the more bizzarre documentation demands, but if anything they have never been as bizzare as they are now.
The only way to contest an AML issue such as a frozen account is through the courts. This has been confirmed by the old Banking Ombudsman when it comes to customers trying to escalate complaints about arbitrary account closures and frozen funds.