JAB,
I agree with marvellous, but does your company operations policy not address this issue ? Lots of ways to skin this cat, I guess.
My old airline ( in the USA ) required a T/O alt any time the wx was below CAT 1 because that was as low as we could go on one engine even though the airplane was CAT3A.
Of course, once the emergency is declared... However, I'm not sure I'd want to be trying a S/E approach to CAT3A if I had a decent altenate within req'd time.
Just a thought...