Here are some "straight from the horse's mouth" websites with more information. Musicians: EDUCATE YOURSELVES!

BMI's FAQ:

http://www.bmi.com/licensing/business/generalfaq.asp

UK specific websites:

The Performing Rights Society:

http://www.prs.co.uk/

Mechanical-Copyright Protection Society:

http://www.mcps.co.uk/

British Academy of Composers and Songwriters:

http://www.britishacademy.com/

British Music Rights:

http://www.bmr.org/html/guide2.html

The Guild of International Songwriters and Composers UK:

http://www.songwriters-guild.co.uk/