Clinical validation of blood pressure monitors in the UK is done by the British Hypertension Society. Their complete list of the currently validated machines is available here:
Because your blood pressure monitor works automatically, it will need to be re-calibrated at least once every two years to be sure it is giving you accurate results. To have your automatic home monitor re-calibrated, you will need to send it back to the manufacturer. There will probably be a fee for this service.
Both of these devices must meet the same stringent specifications for accuracy. Pressure: +/- 3 mm Hg (millimetres of Mercury), Pulse: +/- 5% of display reading. These values meet or exceed the provisions of EC directive 93/42/EEC(Medical Device Directive), and the European Standard EN1060, Non-invasive sphygmanometers Part 1: General Requirements and Part 3: Supplementary requirements for electromechanical blood pressure measuring systems.
The choice of device is basically one of convenience. Some people find the compactness of a wrist blood pressure monitor a benefit especially if one is travelling. Others prefer the larger buttons and display on the upper arm blood pressure monitors (easier to use and read).