I've done a couple of these, with an operation in between.
The first thing to say is that all that has been found so far is some blood in your stool sample. The vast majority of failures are caused by something comparatively minor - piles etc. Even eating really rare meat the night before has been shown to give false positives. And, as parkdale has said, some drugs can set it off.
The thing is, that if there is a problem, the earlier it is found, the less chance there is that it will have developed into something more serious. That's why the whole screening process has come in.
The worst thing about the whole procedure, frankly, is the 'clearing out' process you have to do the day before. When they say 'don't go too far from the loo', they mean it. Believe me!
When you go for your colonoscopy, the staff know how undignified the whole thing is and will try to put you at ease. I was even given a pair of pants with a hole in the back, if you please! You'll be given some relaxation drugs and they'll talk you through all they're doing. It is undignified, but not at all painful.
You won't be allowed to go home for quite a while afterwards because of the drugs, so take a book. And you'll probably have to have somebody to take you, for the same reason.
As I say, the chances that they'll find something are very small. However, if they do, it's far, far better to find it early and treat it then, rather than wait. In my case, what they found was taken out just a couple of weeks later - no radiotherapy, no chemo, because they caught it in the very early stages.
Doing these screenings is saving no end of lives by finding things earlier. But there's only a 50% take-up in some areas. Daft. Take my word for it, you're doing exactly the right thing.
The best of luck to you.