I'm not sure where you're getting March from? The girls' school deadline is 1st October:
For applications for Year 7 to be considered as in catchment, applicants must be able to provide
evidence of their residency within the catchment continuously from 1st October of the year
The statement "within catchment" is a misleading one. You need to have moved into catchment (and thereby have the best possible chance of gaining a place) if you are going to move at all
by 1st October.
If you are out of catchment, you could still be offered a place depending on distance and demand in any given year. The allocations you refer to in the Greater London area occurred because the school was able to offer places to all catchment children and then some. That is unusual when compared to the historic allocation profile, but it happened because for the 3rd year in a row, the school admitted an additional 30 pupils. That change to the Admission Number (PAN) is now permanent.