Surely no agent or publisher would reject work just because you'd used double letter spaces after a full stop, or double quotes for speech? As far as double quote marks are concerned, it doesn't look at all right to me when someone in your novel is quoting something, so that heftier double marks end up inside single ones.
From the (unpublished) work I've seen, at least half of people writing in the UK use double quotes in their manuscripts. (It's much higher amongst those who are writing business-type documentation.) It's the publishing industry that is so fixated on the use of single quotes for speech.
I suspect the deeper reasons for single are:
People in the UK who learned on US keyboard, but can't cope with the transposition of @ and " (even though they have to learn not to shift when typing the quote)
An attempt to standardise as news article headlines (bolded) that include a quote use single rather than double quotes because of ink/space (and single quotes look fine in that instance).