I don't understand.
Here is an English census for a Mary Ann Hallock of London, England.http://search.ancestry.com/cgi-bin/sse.dll?db=1911England&am...
The fifth line from the top of the ancestry transcription shows she is a female. This is deduced from the fact that, in the original record, her age is in the female column. That sex is posted into FTM when the record is merged into FTM.
You shouldn't have to change anything - unless you know the sex is wrong. (And I have seen where the sex is wrong, particularly with children, but even occasionally adults). Then, the sex needs to be corrected.
Ancestry's treatment in censuses, whether US or UK, seems straightforward to me. I don't understand what problem you are having with ancestry's handling of it????