UC Browser has just been updated to version 18.104.22.1680.
The official change log notes faster start up times, smoother browsing and 20% less installation size, but it is the last feature which is most impactful.
The app has long allowed Windows Phone users to download Youtube videos to view later, but did not allow sharing or otherwise using the content.
Now video caching lets you watch videos offline and share anytime, anywhere, including to internal storage and SD cards.
Videos are saved to Pictures\ UC Downloads, from where they can be viewed by any other app and even moved to a different location with the file manager.
The updated app can be found in the Windows Phone Store here.