Well as the author of Scan125 I think I can safely chip in here a little.
Firstly my Scan125 Control Program does not enable any otherwise lacking functions in any of the 125 series of Uniden scanners.
Scan125 was originally written based on the publicised BC125AT api. A lot of reading, detailed specification comparisons helped me, with the help of my UBC125XLT scanner work out what the Scan125 could support in each of the US BC125AT and European UBC125XLT scanners.
Then with the help of Paul (pb_lonny) and his Australian hemisphere UBC126AT scanner I could then make Scan125 support this scanner as well.
Why???????? Uniden chose to create diverse variations of 125 series scanner firmware I have no idea. I suspect that "under the covers" the base 125 hardware is capable of supporting all of the regional variations and it is only the loaded firmware that differentiates them.
I also note that the US BC125AT scanner has received firmware updates BUT the UBC125XLT and UBC125AT scanners seem to have only ever shipped with just one firmware version.
I continue to develop Scan125 with user requests but I'm bounded by each of the regional 125 firmware support command sets.
Finally, as yet I have not discovered any way to make the BC125AT / UBC125XLT / UBC125AT scanners support any of the bands/frequencies/etc. of another 125 scanner that they lack.
What Scan125 can do with regards to each scanner is defined/documented in this table:
http://nick-bailey.co.uk/scan125/manual/Scan125-Manual.html#4