From 6 April 2016, most UK companies and LLPs will be required to keep a “PSC Register” in which they list "People with Significant Control”,

People with significant control are mostly individuals who have significant control over the company or LLP, because they 

This obligation applies to incorporated companies limited by shares (even dormant companies), companies limited by guarantee (including Community Interest Companies (known as CICs), the societas europaea and LLPs.

It does not apply to companies that are official listed or traded on AIM.