Do you have a Raspberry Pi? The United Kingdom was really lucky to have developed such a thing. 
Yes, I have Pi. I'm currently running Raspian but would really like to load Mate. Do you know how I could do that?
You can go to https://ubunto-mate.org/download. However, keep in mind that this can only be installed on the Pi2 and Pi 3. 
That's the site for download, I know that. But after I burn the iso, I don't know how to boot the pi to a DVD.
I'd recomment booting from a USB. You'll need a Micro SD, ethernet cord, and USB storage device.
I have all that. Pi 4 will not naturally boot from usb. I suppose I need to use the terminal/command line to make some sort of change - but I don't know what to do. Do you?
Make sure you follow the tutorial. Certain models of Pi don't support USB boot.
Pi 4 is supposed to. I saw in the forums that some users have rigged a 4 to boot from an ssd. That sounds pretty awesome. I wish I was smart enough to do that. Are you proficient in Linux? 
I am not that proficient. You could try to copy your root file system to an external drive connected by USB. If you tell it to use that file system instead of the one on the SD card it should work. 
I should give that a try, so, Thanx. You seem to know more than I do. You have any Python skills?