Capitals after mdash

I would like to know if, in British English, if there is a question after emdash, do I need to start the question with a capital? for example

—Do you know what this means?

Also, what about a general sentence and not a question after the emdash?

I have the same question for a colon (but of course in not-so-formal writing colons are replaceable with emdashes).

thanks