The refraction certificate examination is run by the Royal College of Ophthalmologists several times each year in the United Kingdom and abroad. It forms part of the FRCOphth qualification. This exam carries a considerable cost, especially if you have to sit multiple times.
The Royal College of Ophthalmologist has updated the format of the Refraction Certificate in 2019. There will be 10 stations instead of 12 with the removal of the trial frame / visual acuity station and near addition station.
Official guidance about the exam can be found on the Royal College of Ophthalmologist website: Refraction Certificate