There is no independent body governing the usage of English. Nor is there a definitive 'rule' book that covers all contexts. As David Chrystal has pointed out, English is a democratic language that has evolved organically, rather than a rules based one imposed from above.
General guides & context specific
Though technically not rules directed, there are, of course, conventions about correct usage. These have been many attempts at producing a universal standard guide, most notably Fowler's Modern English Usage first published in 1926. This remains popular but many have challenged claims that its judgements should be regarded definitive .
Style Guides
In place of a universal rulebook UK major publications, academic institutions and companies have what are known as style guides. The Times Style Guide (2022) for example, offers:
A practical guide to English usage with preferences ... including capitalization, hyphenation and variant spelling... it also advises on terms that should be treated with caution and defines words that are easily confused.
Grammar and the Use of English
Other key reference works apply general rules adapted to specific contexts. A Practical English Grammar by Thomson & Martinet, which is heavily used by students of English as a foreign language.