AnB
Are you sure you can't change wireless channels on your Plusnet router? You should be able to use a web browser and type in 192.168.1.254 that should take you the router's admin interface where you can make changes. (your router's admin password is on bottom of router).
What is the model number of your Roku Express? (found under Settings/System/About). Different models will have different capabilities. If you have any model number other than 3960, you will be limited to using the 2.4ghz band so all changes will need to be made under the 2.4ghz band in the router settings. (select Advanced, and then the 2.4ghz band).
Under the 2.4ghz verify that you are using b/g/n protocols, and under "wireless channels", try using a manual supported channel between 1-11 instead of Automatic. (1, 6, 11 preferred). Verify that you are using WPA2 security. (all these settings will be under the same tab).
Other thing to try is to change the wireless password. (avoid passwords with special characters and try using just numbers and letters). Some older Express models had issues with some routers using special characters in the password. (usually it was the # sign that seemed to cause the most issues, though varies).
Here is Plusnet support site with a few easy to follow videos. (check out the 'how to login to router ' and 'how do I check or change my wifi settings'.)
https://www.plus.net/help/broadband/router-user-guides/hub-one/
Feel free to post back if still having issues. Make note of any error message numbers you are seeing as well. I do not see any indication that Plusnet offers wifi 6 on these routers so I don't think you are dealing with any ax/wpa3 issues. (if you see either AX as a protocol or WPA3, mention that in your response).