I purchased a 3821R recently and didn't receive a correct remote. I don't find 3821 sticks marketed and in stock near me, Best Buy seems to have 3820R which is maybe only a difference in the remote. Will the 3820R receiver be the same? This stick is being purchased for travel in the UK for several months. The 3821R I bought has 3821X on the stick itself and to the right of the HDMI I see UK etc. Will the 3820R when connected in the UK install the correct UK Channels and apps?
From the information I can find online, the only real difference between the two is the remote. The 3821 appears to come with the Voice Remote Pro, which has the rechargeable battery. The Wiki table shows identical specs for everything else. You can use any Roku remote other than the basic IR remote. The Roku Stick players do not support IR control.
You may encounter an issue when traveling to a different country. Your app/channel access is dependent on the country your Roku account was created in and where you are physically located at when using the device. If your account was created when you were on a computer located within the US, you will only have access to channels/apps that are available to US residents. Simply taking your Roku to another country will not allow you access to apps that are specific to that country. It will also mean that you might not be able to use all of your US based apps while located in another country.
To use your example of being in the UK, you will not be able to install the BBC app and use it while you are there. The BBC restricts what devices are permitted to use their service by model number, so only UK specific models will work. That doesn't mean your Roku will be useless while there, as there are some services that don't care where you are (such as YouTube from what I understand).
The intention is to add it to a persons account while in the UK and use a wall adapter from them to connect while in the UK. The Ultra was never marketed in the UK so carrying it didn't seem to make sense. Roku presently in the UK is a hard find in retail outlets.
*** Another question though. I saw a comment a while ago about using an Ethernet adapter for connecting sticks BUT if the stick's remote is WiFi how can that work? I assumed the RoKu WiFi required the correct WiFi USB dongle to function. Can an Ethernet adapter like the SMAYS actually be used with the 3820/3821?
https://www.amazon.com/Ethernet-Replacement-Streaming-External-Internet/dp/B09BFPQ3V5?th=1
The only way to change user accounts on a Roku device is to perform a factory reset. Yes, this will put the device into the UK device bucket, but for apps like the BBC it still won't work. As I mentioned, the BBC has hard restrictions on which Roku device their app will work on, and from past user comments the Ultra is not on the approved list. That doesn't mean that other UK apps won't work, but it's a possibility. I just wanted to warn you in advance. Many providers block access from any device that isn't sold within their market, which is why the Ultra will have restrictions. It's never been sold in the UK market.