I've been using the UT800 with the Edge 25 for about a month now and am reporting on my results. Given the simple nature of the Edge 25, you get limited control compared to the more robust Edge units. The light turns on and off with power on/ power off on the Edge 25, and there's an additional menu option on the Edge screen to turn the light on/off. The most significant issue I've found is that the light has to be paired with the Edge 25. Upon power-on the Edge asks Yes/No about pairing, but you have to select Yes. If you select No the same question just keeps coming back until you do select Yes. Being forced to pair with the Edge 25 was an issue because the button on the UT800 has been non-functional when paired. The light comes on in 800 lumen mode and there was no way to change it. The Edge 25 does display a low-battery message after around an hour and switches the light to 200 lumen mode, so it has been possible to get about 4 hours out of the light before it turns off completely.
But this "dead button" issues has been cured with the new 2.40 UT800 Firmware update. Had to use an ANT stick to perform the firmware update as it can't be done through the Edge 25. The button now cycles through the 800/400/200/pulse/flash/off modes. Have run into some weirdness using both the button and the menu option on the Edge 25. For example if you cycle all the way through the button options turning the light off then on again the Edge 25 may see it as being off and offer to turn it on even though it is already on. And the light does still occasionally just turn off by itself, although other threads here indicate that's an issue with the UT800 in general.