Update: the Disney+ app appeared on the 24th of March, hurrah! Which I'm sure you all know. 
No firestick needed - haven't even looked to see if it's accessible via Prime on, for example, a phone/run away ipad etc.
I have, however, put the app on 2 phones, 2 tvs, and a smart trouser press. OK not the trouser press, but I probably could install it on all my white goods that feel the need to chat online with all the other tumble dryers and boilers and food mixers when my back is turned.
I wasn't going to put it on my phone but now at least I can work my way through The Avengers franchise whilst I'm queueing at Tesco. To get in.
Thank you for all your help, mners, and I leave you with this public service announcement:
to ease your hangover, watch calming documentaries about sea life on, for example, Disney+. This is going to save you money in Alka Seltzer and Berocca, the kids can watch too (which they couldn't when you were easing your hangovers with Killing Eve) and failing that, Finding Nemo (found!) (ooh, soz, spoiler) will do a similar job.