I don't think Sony is leaking this, here's two theories.
Conspiracy Theory:
A Sony marketing team anonymously selling it on an online auction site, not caring who would buy it. Reducing the impact of the unveiling.
While Eurogamer have a history of journalistic integrity, this time they manufacture a Streisand Effect, lying their ass off about fake legal reasons. It was Sony who orchestrated it all, while insinuating the video could cause legal issues to Eurogamer.
Sony makes a secret deal (which itself also never leaked) with only one UK retailer, with early production stock.
Somehow other retailers don't care about not being allowed to do the same, and being stuck unable to sell their remaining stock of fat PS4 because of the leak.
Occam's Razor Theory:
It's obvious this is in full production, and it's been accumulating in warehouses all over the world.
The first could have been stolen from a warehouse or from a store. ("fallen from a truck")
Eurogamer knows it was stolen, so for legal reasons they removed the footage being used for the investigation.
Or Eurogamer went to see it faking they were interested in buying it, filmed the private house, and realized it's illegal to put the footage online without the consent of the person.
If there is a criminal investigation about the theft, it isn't being made public until an arrest is made.
The UK retailer is simply breaking the launch date getting massive publicity, like some often do for games.
The price wasn't announced yet, allowing the shady retailer to sell it at the current non-slim price.
Someone broke NDA, and will get legally fucked.