My view would be it doesnt make any difference.
There will be many pilots out there with good gcse grades, and good Alevel grades, and a good degree behind them, perhaps a masters too, who still have not managed to be given an inteview with any airline!
Where you do your flight training seems to be most important, how much money you spend seems to be directly proportional to the end success!!
Save £100K go to Oxford/ctc, buy a type rating and I dont think your pre flight school grades will count for anything!
However having the grades when you do not get a flying/airline job, and need to earn money elsewhere is when it helps.
Have a back up plan!!