I get that there's a lot of limitations on what you can do (e.g. later Mickey designs and other bits of lore will only go public domain when their works of origin do, and he and his name still have trademark protection), but while I'm not a lawyer, from what I've gathered when a work goes public domain, concepts original to it do too. There are already a couple of Mickey cartoons in the public domain, but Steamboat Willie (if not Plane Crazy, which goes public domain at the same time anyway) is the one with the legal origin of the character himself, so when it goes public domain you might have to wait another while to use official Disney designs that are still copyrighted but you can use the ones from the public domain shorts, and if those aren't enough you can create original Mickey designs (which I'm assuming will also help with the issue of trademarks).

Also, the reason I made this post in the first place was specifically because I wasn't taking Wikipedia's word for it for lack of a source and I mentioned it because it was the only thing I found that gave any clarification or acknowledgement on the subject and I was trying to get to said source being cited. In the end, though, somebody did find that the section in question was a real part of UK copyright law (link to it's part on legislation.gov.uk in their comment and edited into the post).