I was thinking MIT online videos possibly? or anything on youtube? or how about paying a postgrad Physicist at your uni cash in hand for a few hours a week.
I don't mean to sound demeaning; you're probably very clever (ugh i hate that term, but anyway) sometimes students can be very good at the subjects in the category to which they belong - sometimes just *that* particular subject.
I have friends who studied say Classics at some of the best unis (oxbridge, UCL etc) certain friends changed subject like you; history, english, politics etc and they had an effortless transition. They just "picked it" up. But other friends who studied say History and changed struggled.
Same thing with science: in that top percentage of people who come out with top grades in A levels sciences there are two categories (subjective I know) the smaller proportion of students are literally good at every science; you could change them from chemistry to astrophysics, or medicine to engineering and they will just do it. Whereas other top students with straight As will find it hard to adapt.
Others have said you in the top 2 percent or whatever, but if you didn't study physics at A level, and your not one of those student who just "gets it" then clearly you may find A2 equivalent work challenging and certainly first year degree level (at Oxford no less) even more so challenging - there are I'm nearly so sure Physics A level students who *chose* to do physics at A level and now at Oxford who are struggling with first year work. So don't be too surprised that your finding it hard to learn all this physics stuff, at an intense (8 weeks! p/term) rate and in depth; you can do it - I believe in you (though I may not know you.)
Stop with the Olympiad stuff for now. Get to basics. If btw friend are offering to (genuinely) help you, dont feel bad, rather tell them thats how you feel. They will validate your ability and confirm their motivation in wanting to help. We all need a helping hand sometime. If your friends knew how you felt they may have wanted you to tell them. I would if my friend thought they were a burden: I would't want anyone who I offer help to to feel that way!
So i guess i would encourage you to get post grad (or even clever undergrad) to tutor you - is there a physics club where you can mingle and find someone?
Perhaps look at online tutorial, unit by unit - not all at once.
And dont whatever you do become overly focused on getting a 1st - focus on enjoying your course, learning and understanding.
I'm sure you will do well. Tutor (find), tutorials and don't overtly worry about future exam results - use it as motivation not as something to die for; you may actually kill of your interest, motivation and then the achievable first you so want.
hope this helps.