On that device, have you logged into it? Paying for a membership without logging in with the same account will stop you watching NOW.
What membership did you take out?
What are you trying to watch?
On the device app go all the way to the bottom until you see a row of options. One should be “account” in there you will see if any account is signed in. This is where you will need to log in.
As long as the mobile is running on Android 5.0 or later then you should be able to download NOW. Is your Google play account set up for the UK?
Then that’s why you can’t find it to download it. The Google play account absolutely must be set up for UK in order for you to download the app.
If you do a Google search I’m sure that will point you in the right direction to change your “country” for the Google play account. Only then when it’s changed to the UK can you download the app.