Good news! Just finished using the App and have clicked Submit!
I applied online on April 15.
Got the app invite email last night.
So, this morning...
Logged in using email address, UKVCAS password... It comes up with your name and there are two main sections - Documents and ID Verification.
It showed in the Document section that all my docs were already there.
For the ID Verification bit, you have to take a photo using the app. Quite easy.
After that they do a "liveness" check where you move your head around while the app records... (!)
Then you take a photo of your BRP... review and submit.
It's very easy to use. (By the way, I recommend that you watch the helpful video in advance, as it shows you the steps. You find the video link in the FAQ section on the UKVCAS website.)
NOW the next stage of waiting begins. I will post when there is any progress.
Good luck everyone!