With "Optimise iPhone Storage" mode, is it possible to tell iPhone to keep some photos/videos always locally available? I have "iCloud Photo Library" enabled, and "Optimise iPhone Storage" selected (because there's not enough space locally for all photos and videos).

With these settings, is there a way to tell iPhone to keep certain photos or videos always available locally? What now happens when I don't have access to internet is that I can't play videos at all (ones that have already been uploaded to iCloud). 

Especially when travelling where roaming is prohibitively expensive and most of the time I'm without internet, it would be very useful to be able to select some files to be available offline. 

One thing I tried is favouriting a video, but that doesn't affect this. As soon as you're offline, you can't view even a second of your videos.
When using Optimize Storage, iCloud Photo Library does not have a feature to select photos to permanently store on device. If you’d like to send feedback to Apple, you can use www.apple.com/feedback

Personally, I’ve worked around with this limitation a few times by viewing photos I want ahead of time. Granted this won’t work for all situations. However, it worked for me when I wanted to show my grandmother in law my wedding photos. I know these steps work with up to at least 100 photos.


  
  Tap the select button in the Photos App.
  Select photos you would like to view offline.
  Tap share and choose an option such as Mail.
  Wait for the “prepare” overlay to complete.
  Cancel the share.
  


While this works, it’s not a permanent download. If your device begins to run low on space, your selected images may be replaced with small thumbnails.

There maybe other methods to share photos to another app that will permanently store photos on the device. I’ve not looked into this though.