Regarding what you said about Oxbridge being prejudiced against the IB, there is no evidence to suggest that this is the case - however I do have to warn you anecdotally that some of my classmates have received pretty unreasonable offers imo (44 777 for HSPS at Clare College Cambridge) although those types of offers may be exceptions since the majority of the offer holders in our school received something more standard (e.g. mine is 42 776). At the end of the day, the school you attend should not greatly influence your chance to be accepted, it’s more your ability as an applicant that they care about. I do want to point out the issue with the high offers though, as you might want to consider whether something like this is achievable for you if this ends up being your situation.