Miriam Rainsford's article on alternatives to standard publishing contracts for musicians points out the UK problems with the MCPS monopoly:
One difficulty which bLiP records encountered in releasing the Madonna Remix Project album was that MCPS (UK Mechanical Copyright Protection Society) licensing is at present incompatible with copyleft licensing. Weston wished to draft his own license which would permit his customers to enjoy sharing mp3s of the album. When he approached the MCPS, their response was that a copyleft license would be "inappropriate."
On further enquiry it seems that, as an artist signs an exclusive license to the MCPS for royalty collection, the MCPS are then unwilling to extend this license to include file sharing. This is somewhat understandable when one considers that the MCPS takes an 8.5 percent cut from every album or digital distribution in the United Kingdom. The MCPS are willing for their members to use copyleft licensing as long as they sign a waiver for royalty collection. But what made Weston's job difficult is that, under UK law, it is illegal to press an album without an MCPS license. And an album license will not be granted by the MCPS if the album uses copyleft material.