There's a certain set of criteria for schools to be 'prep' AFAIU. DD's school, only opened for five years, is currently a junior school to its senior (over 100yo) but last September was inspected by relevant body and has been approved for membership of some prep school organisation (sorry not at home and it all went over my head a bit). I'm not sure if this means its stationery will now change to read 'XXX, preparatory school to YYY'!!
DD's school is well outside London and offers care from 8am to 6pm. AFAIK, pretty much all the independent schools locally do this, at least until 5.30pm.