It is not the permission of the performer that is required for posting on Youtube, it is the permission of the author/composer (presumably owner of the copy right) of the song. I am sure that permission is required to republish a song on video, and you cannot just pay the license fees. Youtube is video and laws that cover publishing of video recordings are different from laws that cover publishing of audio recordings.

I know US and UK laws differ somewhat in regard to republishing an audio recording - in the US I can obtain a mechanical license fee and pay the fee and composer's permission is not required - but I believe Richard Bridge said that it is different in the UK, where Permission is required. In any case - that licensing law does not cover video. I believe you'll find that in US and in UK permission from the composer of a work that is performed within a video - is required for distribution of the video.

And of course permission and licensing for posting video of unpublished works at Youtube or anywhere else is always required.