Ugh that's annoying. Honestly social media is your best bet - there are loads of resources online and you can pay for some courses (like oxbridge mind) but there are definitely good free resources too!
Could you talk to a head of sixth form or even a class teacher to see if they would help you personally or at the very least direct you to some support? Surely they will have some support for early applications (medicine, vet med and oxbridge) so there must be other students in your year who want to do medicine etc.
My school is a state school so doesn't offer much support but they directed my friend to mock interviews and application support at a nearby private school. Check in your area if other schools or groups offer and get in touch to see if they would help you.
Likewise, if you know anyone that got into oxbridge or have friends/family that have ask them for some tips and advice.
I totally understand that you want to be prepared and well-supported but at the end of the day, it's your application and many people get into Russel group unis without external support. Your school must offer some guidance for general applications, so apply this to your best ability to your own and then watch YouTube videos to understand how to make your application 'better'. I'm sure there's even some threads on here that are directed at keen Oxbridge applicants.
Yes, UNIQ would be helpful but it is in no way necessary and not completing it will not at all jeopardise your application, if that's what you are worried about. Try to get some relevant work experience or other summer schools to go to instead. Equally, attending university open days will be really helpful (even if it isn't oxbridge specific). Ask the admissions officers, students and lecturers you meet on what makes a good application.
Oxford has a big focus on grade so try your hardest academically and do a bit of extracurriculars to write about in your personal statement. My main advice to make your application strongest (with or without UNIQ) is so focus on your LNAT exam, especially if you are wanting to do Law because Oxford places a lot of emphasis on a high LNAT score (30+) which is more difficult than you would think - so start early! Remember, unis will view your application holistically so it is a range of factors which determine whether you get an offer.
Rooting for you!!