You should see the Channels for the UK, if the active* user account has the same region. The listing of Channels will be in the Watch Now tab of the Apple TV app, when you scroll down a bit.
I can’t verify now what the current UK Channels are, but I can’t imagine that Studiocanal would stop so soon after its launch of this service (2 mo.) [1] [2], especially if your subscription is still active.
* Apple TV allows multiple user accounts. Some content may not be available if the ‘wrong’ account is currently active. If this applies, then first switch to the intended user with the subscription for that service.
Try force-quitting the TV app. Then try again.
Try force-quitting the TV app. Then restart the device, and try again.
If the issue persists, then contact the Apple Support staff.