I renewed my class 1 today (only for the 4th time in my life!) and suffer from the same white coat syndrome. The trouble is I start to worry about the BP thing about a week out (I don't know where the initial worry came from to be honest!) and the anxiety builds up.
They bring out the machine and I can feel my heart trying to escape from my chest in utter defeat. It feels very much like an adrenaline rush to me (Fight or flight type stuff). This gives me an initial reading of something like 145/89 today. The next reading after me concentrating on a my book for a bit and generally trying not to think about it is down to 127/79 (still with a 102 heart rate) and then it keeps on coming down toward the optimal reading and a normal heart rate.
It's completely illogical but I have never been hugely keen on tests and am early in my career so everything seems like a fairly big deal. Our AME is lovely and sees the panic. As has been mentioned previously, for me it's suddenly a sense of having no control over the outcome of something quite important that does it I think.
I regularly self monitor for my own peace of mind (as important as my career is to me if something is wrong with me I'd rather know about it) and am always within the "optimal to normal" range.
I think a good AME know's when they're looking at a case of hypertension and when they're looking at a prize prat like me who is scared of an arm cuff with a pump attached.