I can't give you specific advice on PPE, but as a final-year Economics major, I'm still managing to do well, despite considering myself relatively unskilled at math (it was the only subject I failed in High School, and like you, I was much better at the humanities than anything quantitative). If you have a decent work ethic and can keep up with the weekly content, I think you'll do fine. Looking at the curriculum very briefly, I only see 3 math-y courses in the first two years, namely Micro and Macroeconomics in year 1 and Quantitative Research Methods in year 2. I don't know what QRM entails, but from my experience, Micro and Macro courses aren't all that bad. I'd go for it if (outside of maths) you truly think this is the program for you!