Huh, I looked this up and you're right. It's also not available on other European Amazon stores. That aside, I'm from RO and it's stocked here at almost every retailer I've been so far which also carry physical games so it's weird that it's not the same in other European countries.
As for Flanco, they're a legit retailer chain and a subsidiary of Emag, the biggest online retailer in the country (they're basically our version of Amazon), but I'm not sure if they ship outside of the country. See if you can import it from France or Germany or closer from
Bulgaria.
And speaking of Emag they also have the HFW Complete Edition in stock:
https://www.emag.ro/joc-horizon-for...pentru-playstation-5-1000040784/pd/DVCT12YBM/ (see if you can get it from here as they have good return policies).
Oh and another option is
PlayStation Direct but I'm not sure if they ship across all Europe or just in the UK.