This problem happened to me yesterday when I renewed my medical. Two days before I'd had my BP measured at my GP's surgery by the nurse (amply proportioned everywhere!!) which gave a reading of 128/75.
Yesterday, the AME clamped the cuff round my arm and I could physically feel myself tensing up. The result was a BP of 155/80.Technically, the upper reading was above limits, but I explained the lower readings recorded two days before, and the Doc. passed me! I guess the AME can always check my story by phoning my GP's surgery if he feels minded to.