Have you looked at any of the settings/info in the "Platform Secret Screen" (press Home 5x, then FF, Play/Pause, Rewind, Play/Pause, FF at about 2 button presses per second)? It might be interesting to see what channel it's using when you have it connected via a wired connection, although you may need an IR remote (which may be the solution to your troubles anyway) to access the secret screen. For what it's worth, both my Roku 3 and 4 that use a wired connection are using channel 165. If some 5GHz channels aren't available in the UK I would think the firmware wouldn't try to use them, so could it be that the unit thinks it's in the U.S.?
telnet to your Roku's IP address port 8080. You should get a prompt. I'm not sure whether or not the Development Application Installer has to be enabled for it to respond on that port, so if you don't get a prompt or can't connect you may be out of luck since it takes another remote control key press combination to enable the installer.
At the prompt type:
That emulates the same sequence as I gave above.
Edit: I forgot, you can get your remote working when you have the Roku connected to your 2.4GHz network. If you need to enable the Developer Application Installer, connect to your 2.4GHz network and use the following remote key press combination: Home 5 times, Up twice, Right, Left, Right, Left, Right. Pretty much do it as fast as you can and ignore anything happening onscreen. You can leave the installer enabled; it doesn't hurt anything.