If what you say is true, Steamboat Willie, as well as Fantasia are both out of copyright in the UK. I wonder why no one has started selling copies?
The first time I saw the whole "50 years on fixed performances", I went "YAY! I can put them online!" Thankfully I talked to a lawyer who told me that the script and music rights are transitive and _not_ extinguished by being embodied in another work.
Of course, my IP lawyer might have been wrong. Personally, I'd love for you to be right. How about you put something up in the UK and see what happens?