This slim volume, now in its 39th edition (1983, corrected 1989, my copy reprinted 1998) is about as close as there is to a British English equivalent to the American's Chicago style guide. If you are uncertain about when (and how) to use an em-dash or whether roman à clef should be italicized, or whether the correct plural is bravos or bravoes, this will see you sorted. As is reasonably well known the OUP has a number of idiosyncrasiesit is a bastion of -ize spellings and the Oxford commaand these need to be taken into account. Its focus being academic, it includes absolutely all you need to know about attribution and quoting styles as well.
As its name hints, the book was written for professionals in the hot metal era, and it contains quite a lot of stuff about picas and em quads rather more technical than the average Microsoft Word useror, come to that, those who write HTML straight into viwill need to know. It also has sections covering other major languages (most of those written in Roman, Greek or Cyrillic script).
If you want to write British English, this is a tenner well spent. The only widely available alternative in book form is the relentlessly mid-Atlantic Economist style guide; the English division of the European Commission's Translation Service also has a fairly substantial on-line offering at