Hi! I've actually changed my offer from Law to PPE as I realised that straight law isn't for me upon taking a MOOC, and I enjoy the interdisciplinary nature of the latter course more. (though disclaimer: I am not sure whether I made a right choice) It really just depends on your choice and what you're interested in. A question that I was faced with primarily by my teachers is: it's a lot of effort and time being put into your degree, will you enjoy the choice you've made?
However, in terms of joint law, I've heard advice that a Law degree is already extremely rigourous, and a joint honours degree would make the difficulty harder. I would take this with a pinch of salt.
Maybe consider the pros and cons of both, and go through the module list along with the differences? The choice is really difficult, and I hope that things work out!