When I come to re-save old files that I've opened in Photoshop CC it will frequently say that I don't have the necessary permissions and that the file is locked. This is absolutely infuriating and I would like to turn this function OFF entirely. Can anybody please help
That means that there is a problem with the permissions on your disk, or that some other application is using the files on disk and won't let you overwrite them while they are in use.
Photoshop is just reporting the error that the OS gave. The problem is not in Photoshop, but on the disk where you are trying to save, or in another application holding the file open while you are trying to save.
To solve it, you have to either fix the permissions so you can overwrite the file, or get the other application to close the file so it can be written.