I'd only recommend it if you're sure you'll get a good grade. Oxford is very slightly lenient with Further Maths (unlike Imperial where it's pretty much required), and since your school doesn't offer it it may not be an instant rejection. In the MAT results report (especially the recent ones), they list how many people applied without Further Maths and got offers. In 2019 for example, 74 out of 1427 applied without Further Maths, and 'fewer than 3' were offered a place. However if self study and end up with an A, it might look even worse. They'll want an A* if you do decide to take it. Can you not find a school for Year 13? Self studying for a whole year in general sounds risky