I've always taken time off for appointments from my holiday entitlement where possible, not because I feel oblidged to, but because I'm quite private and don't want to deal with all the questions.
I'm taking 5 days self certified sickness with my op, then a weeks holiday.
They think I'm going into hospital for something to do with my arthritis, that way, when I'm eating healthily and losing weight, I'll just say it was recommended to help my condition. It's no one's business at work really, but in our office, you can't trust anyone to keep anything quiet, so I'd rather they not know.