Beth, I can't speak for your country, I can only speak for what I am familure with in my state and how things are done here in the past 9 years that I have been practicing as a nurse.
Yes sometimes a patient might be exposed during surgery after they were put under, but not usually, and never if it can be avoided. From what has happened to me personally, and from what I have seen as a nurse, very very little is ever exposed.
When an area does have to be exposed it's usually the part being operated on, in which case the area is exposed, and then quickly draped accordingly, and as quickly as possible. They leave as little skin exposed as possible for numerous reasons.
Yes, it is true that sometimes you wake up in a different gown than you went in with, and again, from my own personal surgical experiences as the patient, and my personal experiences as a nurse watching, when a new gown needs to be placed, if the nurse changeing was taught the correct way to change a gown. The old gown is untied and loosened around the patient. The new gown is rolled up and tied on, then unrolled down the patients body as the old gown is rolled out at the same time so that when you get down to the end, the new gown is completely covering the patient, and the old gown has been rolled up and is then placed in laundry. So as you can clearly see, the patient still isn't or shouldn't be every full excposed.
Remember some of the most basic rules for surgery. Sterility is number one and is of top priority. Too much exposed skin is a germs playgreund. Having correct drappings done prior to suregery is a mush to keep out infection control