Hi, A level Art teacher here!
It's not easy to find out because it's rather holistic, however straight down the line, the final exam itself (the work produced in exam conditions) I guess would be 10 of the 40% and a quarter of the grade because it's all of AO4 which is one of 4 assessment objectives.
The prep work is 3/4, however, the prep stuff has to relate to the final piece. Lets say your child chose the theme "still life" and prepped and experimented with still life then drew a portrait in the exam...that's gonna tamper with grades for AO1 and 2 in a way that it wouldn't if the final piece didn't exist at all (if that makes sense), because the prep work is now not worth the prep.
AO2 specifically says that the experimentation should lead to the best outcome possible. If it's felt that your child made a dodgy choice on the day of the exam that will also affect the grade.
AO1 is about research so in my still life/portrait example, the research needs to relate to the outcome. No sense researching vanitas if you're making a Lucian Freud style outcome.
Let me know if that helps!!