When Microsoft released the Microsoft Band it was US only and Microsoft restricted the apps to the US only devices. You could get round this by switching your Windows Phone to a US region or sideloading the app on Android but this was a real pain. Ahead of the launch of the Microsoft Band later this month Microsoft have released the app in the UK app stores.
The Microsoft Health app is available for Windows Phone, Android and iOS in their respective stores and if you are planning on getting a Microsoft Band you can get the app now ready for your new device.
The apps have also been slightly updated, the Windows Phone version is now 2015.325.1953.5399 and has bug fixes. The Android and iOS version has fixes for postal codes and date issues (presumably switching to the correct region formats).
Are you thinking of getting a Microsoft Band?