Em dash in British English

The rule Hyphen, n-dash and m-dash identifies an n-dash separating words as an error. This is not an error in British English, where strong interruptions are punctuated – like this – with an n-dash and spaces either side. Any idea how to fix the rule for British English?

I’ve looked it up, and the few sites I’ve found say that it’s merely a ‘house-style’ popular with UK newspapers, not ‘proper’ grammar.

I have to disagree. It is standard style for Cambridge University Press, at least here in the UK. I also just randomly picked up some books and found that Dawkin’s Magic of Reality and Wittgenstein’s Philosophical Investigations (Wiley-Blackwell) both use this convention. I have always thought it standard for British English.

Also see this site.

You made me pick up again Dawkin’s “The Greatest Show on Earth”, edited by Black Swan, UK.
I don’t find this rule particularly relevant, but this book only use dashes and hyphens. Generic dashes.
See page 10 for examples dashes, that use to be referred as m-dashes, in this context.
Then see page references in the bibliography. The page numbers are separated by a dash with the same length, where generic rules say it should be an n-dash.

If a writer/editor does not use a font that has both forms, or disregard this separation, this rule can be disabled in the options.

Yes I just picked up the very same book (Dawkin’s fans, obviously). Same British convention as you point out. I’ll probably just disable the rule for now.