Currently the lock button only appears in the UI for LFS files (regardless if they have been registered as lockable).  However, the documentation for LFS locking includes a use case for locking non-LFS files:

If you'd like to register a file type as lockable, without using LFS, you can edit the .gitattributes file directly:

 
 *.yml lockable

Doing this gives the correct file permissions client side, but unfortunately does not provide a button for the user to lock files within Bitbucket.