It's always difficult trying to ascertain the relative difficulty of getting a place at different schools.
One resource I found helpful though was the statistics of places offered from a local prep school (http://www.radlett-prep.herts.sch.uk/ag.html
). Obviously it's not very scientific and there are a lot of "ah, but have you considered...." variables, but it's possibly better than nothing. Using the simple numbers from this page, 6/13 (46%) got places at Habs girls, whereas 8/12 (67%) got places at St Helen's. So St Helen's appears to be quite a selective school, but possibly not as difficult as Habs i.e. not getting an interview at Habs doesn't mean you won't get one at St Helen's.
On that positive note - good luck!