There’s no legal limit in the USA either. You wanna by a jet for $10 million using a truckload of wrinkled pictures of dead presidents, that’s not illegal. Nothing wrong with it at all. A tad unusual, but totally OK.
But the seller needs to report the big cash transaction to the US anti-money laundering authorities. Failing to do the report is illegal. The penalty for non-reporting is on the seller, not the buyer.
The authorities will probably do nothing with the report but stick it in their database. If the buyer has a habit of making these big transactions the database will eventually notice and the buyer will be investigated. Not for making illegal purchases, since all these purchases are fully legal, but rather for how exactly they’re coming into quite so much cash so often and does that align with their tax records? The seller(s) meanwhile will be beyond scrutiny having done the required report(s).
I certainly do not know the UK’s anti money-laundering laws, but I imagine they have some way to keep some tabs on big cash.