Well really you should do your work experience in the department you would like to observe. So if you want to see a coronary angioplasty, then you need to be in the cardiology department etc...
Usually, once you're on your work experience, you can get to see different things just by asking nicely. I did one of my work experience placements in ITU and asked if I could see some surgery. So one of the consultants phoned up a colleague and I got to spend a day observing ENT surgery.