I chose it because my GP recommended that in his experience it was accurate, and in addition to testing it against two brands of normal upper-arm cuff (consumer) devices that a friend and a relative had, I also took it when taking my dad to his GP who the previous time had expressed interest in the wrist-based device I'd mentioned using on my dad, and she tested it against her two devices (one an electric upper arm cuff, the other an old-school manual upper arm cuff that involves squeezing a rubber thing on the end of the hose to inflate it and a manual clock-style dial for readings).
We didn't detect any unreliability in it compared to any of those 4 devices (two consumer, two NHS-approved & doctor-owned) - all 5 seemed equally reliable (based on our not exactly lab-quality testing, but still we weren't lazy enough to do single readings or anything like that).