You must hold suitable immigration permission which permits study in order to sit an examination, even if you are only entering the UK for a short period of time. The type of visa you will require is a Short Term Student Visa This visa category is suitable for those student entering the UK for a short period of time to re-sit an examination (or examinations).
As soon as you return to the UK please present your new immigration permission (your Short Term Student Visa)at the student Registry Reception prior to taking any exams.