BP limits are 150/90 for class 1. Anything in excess of that won't necessarily deny you the issue of the medical, but will delay its issue until you can demonstrate that your normal BP is within these limits.
White coat anxiety is a pain, and much like Heathrow Director, i too don't get along with the blood pressure machines. As mentioned, the AME's are well aware of this! I suggest that when they strap you to the machine, completely relax your body (slouch to the point where you are barely sitting in the chair - so much so that even your head is hanging limp) and control your breathing - deep, slow breaths. Smile and think of something that makes you happy.
All the best.