There's a site called camelcamelcamel that keeps a price history of things on amazon. It's great for checking if something was actually discounted, or just marked up a bunch in the weeks before amazon day only to be "discounted" back to its normal price. Last year when I bought stuff for prime day it turned out I actually saved (lost) -$1.73 compared to if I'd bought a few weeks before or after the "sale"
No prime day deals in the UK , as far as I can see (although I did buy some kids' books on coding which happened to crop up whilst browsing for stuff unrelated to prime day; apparently, there's 20% off a lot of books, but that's the usual dubious claim).
Best deals occur throughout the year and are dependent on factors other than trendy fads. Mr. Washington mentioned some and I've spoken of others previously, here, but I should also now include listing errors and other languages ( I recently got Munster Go Home on Blu-ray as the German version, albeit with the full, original, English soundtrack; it's available in the UK and the US and the picture quality isn't too bad for an old film; search 'gespensterparty'. That's my plug for the day.)