Here is what Cambridge says about the matter:
"Only a few of our successful applicants have offered just one facilitating subjects. For these people, their A-levels have usually included subjects that are particularly relevant to (but not requirements for) their chosen course, such as Economics, Music, Politics, History of Art or Religious Studies."
I would strongly advise you to consider doing another facilitating subject. And if you do not do so, try to do something like Philosophy or Politics that will still teach you imperative essay writing skills and look okay to RG universities.