Unfortunately i dont know anything about the UK version of this App. Therefore i cant give you detailed information.
But the Ledger app has not been taken off the Appstore in other countries as far as i know.
You could either search for the App online and verify signature after downloading it (to be sure you have the original, and not modified, version).
Or you use your security card which should be delivered with your Nano.
If there is no security card this probably means you bought your Nano from a non-official seller?
If thats the case i would not use this Wallet.. cant be sure if you got a manipulated firmware.
But if you just lost your security card i think there is a tool which you can download which creates a new security card based on your ledger nano.