We’re talking about different things.

All movements of money above £10K have to be reported to the government.

Cash withdrawal limits do differ between banks, but this is nothing to do with tax, that is more to do with money laundering.

Yep, depends on the bank somewhat. I was transferring ISAs around last year and boy did I get grilled. Wasn’t even cash withdrawals (Twas between banks online). Closed the offending bank immediately because the questioning was pure overreach. As for cash, because the “tracking” is lost upon withdrawal, they really make things difficult. Urrgh makes my blood boil. MY MONEY!