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 e.g. —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 colon are replaceable with emdash)

thanks

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