My OH is a literary agent (not non-fic, soz) and says, sure, an agent/publisher normally wants to see three chapters and a synopsis first but if the writer has already written all the other chapters too that’s not, in itself, a problem. But it is not going to short-circuit editorial either.

From the writer’s point of view there are jolly good reasons why you wouldn’t submit a proposal in the form of a completed manuscript but if you do and it happens to be just the book someone is keen to publish then happy days all round, no?