ISRC codes - http://www.ppluk.com/I-Make-Music/Why-Should-I-Become-A-Member/What-is-an-ISRC/ - tells you what they are and how to get them. If you are not a member of PPL and just require an ISRC, you need to register online and select the 'ISRC only' option.

See also: http://www.ifpi.org/content/section_resources/isrc.html

Wikipedia: "The International Standard Recording Code (ISRC), defined by ISO 3901, is an international standard code for uniquely identifying sound recordings and music video recordings." more...

"How do I request an ISRC?" - and see "How do I request an ISRC?" down at the bottom.

PRS for Music - http://www.prsformusic.com/

Wikipedia: "PRS for Music, (formerly the Performing Right Society) is a royalty collection society that was founded in 1914. The organisation was formed in 1997 as the MCPS-PRS Alliance to bring together two collection societies: the Mechanical-Copyright Protection Society (MCPS) and Performing Right Society (PRS). The PRS for Music brand was adopted in 2009." more...

MCPS - http://www.prsformusic.com/Pages/Rights.aspx

Wikipedia: "The Mechanical-Copyright Protection Society (MCPS) are an organisation who pay royalties to composers, songwriters and music publishers when the music they have created is sold. This includes sales of the music alone such as CDs and downloads, and also products which use the music as a part of their soundtrack, such as films and computer games.
MCPS now sits under the broader PRS for Music brand." more...

UK Music - http://www.ukmusic.org/

Wikipedia: "...UK Music is an umbrella organisation which represents the collective interests of the production side of UK's commercial music industry: artists, musicians, songwriters, composers, record labels, artist managers, music publishers, studio producers and music collecting societies." more...

AIM - Association of Independent Music - http://www.musicindie.com/

Wikipedia: "The Association of Independent Music (commonly known as AIM) is a non-profit trade body established in 1998 by UK independent record labels to represent the independent record sector, which constitutes approximately 25% of the UK market. AIM currently has over 800 record label and distributor members." more...

The Unsigned Guide - http://www.theunsignedguide.com/index.asp

Wikipedia: "The Unsigned Guide is a contact directory for the music industry. Founded in 2003, the publication lists a wide range of contact information containing a contact name, email address, website, myspace, postal address and telephone number. Every listing also includes a paragraph of relevant information about the company or organisation." more...

BPI - British Phonographic Industry - http://www.bpi.co.uk/

Wikipedia: "The British Phonographic Industry (BPI) is the British record industry's trade association...Its membership comprises hundreds of music companies including all four "major" record companies (Warner Music Group, EMI, Sony Music Entertainment, and Universal Music Group), associate members such as manufacturers and distributors, and hundreds of independent music companies representing literally thousands of labels." more...