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I think the simple fact was that women who did resort to prostitution to make a living could not put that down on the census form, so they chose an occupation which didn't involve leaving the house much.  However, to assume that every woman who described themselves as dressmakers were prostitutes is plainly absurd, because before the industrial revolution a substantial part of the clothing industry was done by homeworkers.

That's a point well made, Nick.  I can recommend the website http://www.visionofbritain.org.uk/, which contains all the census reports from 1801.  The General Report from each census breaks down the population by occupation.  Many of the occupations stated by individuals were "standardised" by the enumerators, and then classified by the census clerks according to strict guidelines (they derived a large dictionary of occupations for this purpose).  Very few people working on farms would have called themselves an "ag lab" for instance - this was a term invented for the purpose of the census.  In 1891, over 400,000 women were listed as milliners, dressmakers or staymakers (as well as 4,000 men).  Whilst, I am sure, these terms would have been used euphemistically in some cases, they would have described the woman's occupation correctly in the vast majority of cases.
Going back to the census report, the point was often made in the reports that people exaggerated their occupations.  In one of the reports, for instance, it states that commercial travellers (usually a middle-class occupation) were often no more than hawkers.  I can find no similar suggestion for dressmakers.