I just went through this process - getting my CPL current again after 6 years of not flying.
After calling the CAA I was told that the law exam must be sat for the license held. i.e. If you hold a CPL then the CPL law exam must be sat and the CPL BFR must be done, even if using it for PPL privileges only.
To be honest after that long not flying I was glad I did the exam as there was a lot I had forgotten.
Hope that helps.