Do you have high BP all the time, is this a medical condition or is it/was it affected by stressful situations or anxiety?
I am not at all medically qualified so take what I say with a pinch of salt. But, I have had high BP and I lowered it by reducing consumption of coffee, down from 12 or more cups per dray to less than 5 usually, and beers.
Also when I went for my operation and my BP was taken they were contemplating sending me home as it was so high. I had been taking my BP every day for several weeks and produced these records showing that usually it was around normal. I thought and the med profs agreed that the idea of major surgery that day might be likely to raise it, so my op went ahead.
Other things you might consider to try to combat raised BP is to get to an appointment super-early, thus alleviating worrying about arriving on time. Also, arriving early allows any anxiety caused by the travelling itself to dissipate. And when the BP is taken, sit stretched out without arms or legs folded, apparently this also works.
If you have a BP problem it is better to find out pre-op rather than on the table though.