Maths is useful but not essential. Basically doing PPE you do all three subjects in the first year, then most people pick two. If you don't do A level maths, you may struggle a little with economics in the first year, but you can drop it after that. So while taking maths is definitely recommended, as admissions tutors like to know you can do maths and write essays, it is possible to get in and cope with the subject without it.
If you really want to do PPE, your best bet is to take maths, history and any two other decent subjects and get As.
However, if your goal is a pupilage, why do PPE? Law is as respected a degree at Oxford, means you can skip the (expensive) year doing the GDL or CPE, and doesn't mean you have to take maths A level. Moreover, it'll give you a broader background to law than just having the GDL or CPE. What makes you set on PPE? It is a great subject, and if you're passionate about the three subjects by all means take it, but if you don't like maths so much and want to be a barrister, it doesn't seem to make as much sense to me.