Solved: Britbox Can't Play - Roku Community
Not sure about BritBox in particular, but Roku hardware can be quite finicky. I returned one a few months ago. Which device do you have?
Right off I'd tell anyone contemplating using Roku to consider their more expensive device, the Roku Ultra. Mostly because it's one of the few devices that allows both 10/100 Ethernet and more importantly it allows 2.4Ghz/5.0Ghz (N & AC) dual band capability. In other words if you're router can handle 5.0Ghz using AC protocol, use it to make the most of your Roku connection. 10/100 Ethernet isn't as fast as 100/1000 Ethernet, but at least the option allows for more consistent bandwidth rather than to depend on wireless.
One thing that spooked me about the smaller Roku sticks was the amount of heat they draw. Another thing to consider is using your wall plug for power rather than relying on a USB port on your smart tv to power the Roku device. In any event it's best to elevate your Roku device so it's not sitting directly on a surface. Give it some airflow above and below it. Something I do with all my electronic components so close together using steel shelves for an entertainment center.
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With particular attention to my cable box which is on all the time and can get quite warm. Truth is, I even use a little 4 inch Holmes fan on my router and modem at the slowest, quietest speed. Keeps both components at room temperature. Hot components can potentially slow down or interfere with your Internet connection speed as well. Given the heat that Roku stick put out, if I get another Roku device I may place a small fan near it as well.
Roku
Another thing more complicated is thoroughly analyze your broadband Internet connection, both wired and wireless. Make sure you are getting the most out of your modem, router and are using current RG6 coaxial cables to give you optimal power.
Seems most learned folks agree that for basic streaming of movies and videos you should consistently be getting between 53 and 100 mbps. Anything less and you get a choppy presentation that may be constantly buffering. Ideally you should only get buffering at the beginning of a download and not much else. The Roku wireless stick I bought and returned was giving me a pitiful amount of bandwidth having to use the 2.4Ghz band. Not acceptable!
Consider downloading from the Microsoft Store a freeware app called "WiFi Analyzer" by Matt Hafner. It's a tool that monitors how strong or weak your wireless signal may be, and it also shows your neighbor's connection so you can try to use an optimal channel which hopefully has the least amount of users on it. This program allowed me to optimize how my router and network adapter antennas were pointing, apart from where to position my router relative to my computer's network adapter. Truth is, there's a lot that can hamper your wireless signal that you may be unaware of.
Another thing to do is to run a speed test on your Internet connection through Ookla:
Speedtest by Ookla - The Global Broadband Speed Test
I recently discovered using wireless AC 5.0Ghz gives me relatively consistent speed using channel 48 and 40Mhz channel width that was done by changing my router's settings. There's really quite a few things to consider if your broadband is much less than what is advertised or that you're paying for. I pay for 200mbps and almost always get well above that in the speedtest. But it took a while to figure out how to optimize my router using 5.0Ghz. Though with a Gigabit Ethernet adapter built into my computer I can get a whopping 480Mbps. Not very useful though since my ISP only allows 200 Mbps on my plan. Oh well
Optimizing 802.11ac Protocol Signal Strength