U.K. consumer rights group 'Which?' is filing a legal claim against Apple under competition law on behalf of some 40 million users of iCloud, its cloud
That is only true if other apps have the same operating system access as iCloud. If others apps can't perform the same actions because of vendor lock in, that's anticompetitive monopolistic behavior. Apple already failed to dismiss identical lawsuit in US, so the lawsuit is at least valid on its face.
Issue is that 3rd party apps doesn't have the same system access as iCloud. So you can't use any other cloud the same way you can iCloud. So by definition that's anticompetitive, since you have no options.
Imagine you want to store your iPhone backups on Dropbox, Google drive, or whatever. They're just files, should be simple right? Well, since they lack the access/apis necessary to facilitate this, you're required to have icloud storage if you want phone backups. These include far more than just photos/etc. Idk how I feel about it other than the obvious "I think choice is healthy, apple is capable of designing system compatibility, and I would like to have more choice even if I'm not inclined to leverage it."