Complex sentence: London arose from the split in ownership between the British branch of Decca Records and that same company's USA branch; the American London label released British Decca records in the USA, since it could not use the "Decca" name there.
Split into simpler sentences: London arose from the split in ownership between the British and American branches of Decca Records. The American branch released British Decca records in the U.S. since it could not use the "Decca" name there.