There are both mandatory and advisory sources of generally accepted accounting principles (GAAP) in the United Kingdom:
Company Law - The Companies Act 2006.
Financial Reporting Standards (FRS) issued by the FRC - see 'old UK GAAP' and 'new UK GAAP' below. Listing Rules of the London Stock Exchange and AIM rules.
Statements of Recommended Practice (SORP) Financial Reporting Standard for Smaller Entities (FRSSE)