Hi I’m also a year 12 rn and kinda regret taking geo aswell lol. Similar to you, I’ve been looking at my options (but looking to take up bio not physics) and there are quite a few: pick up physics AS in year 13 and stay on an extra year to get the full a level, take a gap year and do a level physics during it, try and learn AS physics in summer in time for year 2 content (this one is a bit ambitious). When you say you want to do it privately, do you intend on learning it outside of school now alongside your other three? If you’re interested in engineering, civil engineering would suit your a levels as geo is actually quite relevant to it. Good luck!