It definitely sounds anxiety related if you are getting a reading of 122/79 (lovely and normal that) with your Dad's monitor away from all doctors and surgeries and if just thinking about it sends it up by a fair bit.
It would have been pretty amazing if you'd have had it taken yesterday and it had been ok bearing in mind how stressed you are about it a) being taken and b) the op being cancelled if it was too high.
Over the weekend try breathing in and out very very slowly and focus on keeping your breaths even. Unclench your muscles - esp your jaw, fists, upper back. Search out some guided relaxation vids on youtube and lay down on your bed if you can while you listen.
Avoid tea, coffee, cola, chocolate and salty food such as ready meals/bacon/sausages etc. On Monday as you are sat in the surgery before your bp is taken, do the deep, slow breaths in and out and consciously unclench your muscles one body part at a time. Just focusing on doing that can calm your nervous system down a fair bit.
If your high bp readings are due to anxiety and generally when in a medical setting then it just needs checking every so often away from the surgery or by having a 24 hour monitor fitted to make sure it isn't generally too high or fluctuating wildly from low to high which can also be a problem.
I've had essential hypertension (ie they can't find out why) since I was in my early 20's and have taken a cocktail of different drugs for the last 30 years. I can't get mine down to a safe level without drugs but I can lower the readings a bit before I have my bp checked by controlling my breathing etc.
Good luck for Monday. Think calm thoughts!!!!!!