>Do you think that such a high score guaranteed you an offer or do you think the interview was still important please?
Oxford publishes admission information every year for physics (
https://www.physics.ox.ac.uk/study/undergraduates/how-apply/physics-aptitude-test-pat/physics-admissions-reports), and for the past few years it's stated that post-PAT, the interview weighs about twice as much as the PAT, so a good PAT score helps but doesn't guarantee.
Oxford does some access programs for the PAT and physics beyond the syllabus too. There's the Preparing for the PAT course starting in June, plus also COMPOS and Quantum Club runned by Lvovsky (who's also infamous at Keble for his 3 hour tutorials). Great not only for physics but also finding similar-interest peers! There's also the Senior Physics Challenge Residential at Cambridge. Some access programs I can think of off the top of my head right now.
As for the school not doing Olympiads, my advice will be to just chase them because the only way to participate in UK physics Olympiads is to sign up through the school. All schools in the UK should be able to do them, it only takes a couple hours every year each time, the teacher literally doesn't have to do anything except get the papers on time and print them and sit for a couple hours invigilating and scan them in, there is no good reason to not participate. Participating in the British Olympiads is also the only way to be able to get into the national UK team for international Olympiads. Plus if your son make it past round 2, you get to go to Oxford for a couple days for BPhO camp and do fun stuff.
There's online Olympiads too your son can do for fun, like Physics Brawl, OPhO and the Physics Cup.
Speaking of physics Olympiad stuff, I find the advice here to be very helpful (it's intended for people in the US, but physics is still physics wherever you are):
https://knzhou.github.io/writing/Advice.pdf