When I first hit a blocked page on my home broadband I assumed my router was broken, but it turns out UK IP blocking is usually done deliberately by your internet provider. UK ISPs restrict access to certain sites and streams under a mix of court orders and voluntary schemes, which https://en.wikipedia.org/wiki/Internet_censorship_in_the_United_Kingdom describes in detail, covering everything from copyright injunctions to child-safety filters.
The most common trigger for streamers is the live blocking orders that target unlicensed sports and TV streams, especially around big football matches. Your ISP matches the IP addresses or domains on the order list and simply refuses the connection, so the page times out or shows a warning splash instead. These blocks are dynamic, so a stream that worked on Saturday can fail on Sunday.
The EFF tracks this kind of network-level censorship around the world, and https://www.eff.org/issues/censorship is worth reading to understand the difference between a court-ordered block and your ISP over-blocking a legitimate site by mistake.
If you suspect a false positive, check the site on mobile data first, then ask your ISP or use an independent block checker. And when you are setting up a legal viewing app on a stick instead, a walkthrough like how to install IPTV on a Firestick covers the Downloader-code setup side, so you can tell a setup problem apart from a genuine block.