Buying a car before or after you pass?

I’m insured on my dad’s car already, but really want my own car so that I know as soon as I pass I can drive about. Does anyone think it’s worth it to buy it first or wait? My parents want me to wait (I’m 18 about to do alevels) and apparently after A-levels they may buy me a car if my grades are good but that is a good 5months away. Could I also switch my learner insurance onto my own car? Thanks