AB does percussion exams see
hereTrinity Guidhall see
here - drum kit exams
and
Rockschool see
hereDaughter's experience has been that TG exams are more orientated towards reading the music, Rockschool more improvisation / fills required. You may have to travel some distance for TG - they have specialist examiners for drums.
Both boards publish books with CD backing tracks and all the requirements for the exam. TG have two grades per book.
Hope this helps