Perforation is pretty rare, although certain people and people with certain conditions can make it more likely.
Being an English person, I know you've had proper sausages before. Think about how hard it can be to tear the casing on those sausages with a fork or even the typical table knife. That's the same for your intestines; they are rather elastic, and are not easily perforated (a hole poked through them) with the flexible scope that they use. It's really only people with certain genetic conditions or certain intestinal diseases that have thinner than usual intestines or more brittle intestines that are at a higher risk. Oh, and malpractice; a really bad doctor can always cause a problem. As far as I am aware, though, you cannot get a perforation after the scope, because there's nothing in there to poke a hole through your intestines.
Your biggest concern should be diarrhea after the procedure. Most people stop having diarrhea after they quit taking the laxative and start eating solid foods again. However, some people do not adjust well to all of that laxative--see, it flushes out a lot of the good bacteria in your intestinal tracts and some people don't recover naturally and continue to have diarrhea problems. In that case, you should take probiotics for a week or three to get your intestines normalized again. If you have diarrhea all the time as a symptom of your IBS, then go ahead and take the probiotics just in case they help make the diarrhea less. The worst thing probiotics can do to you is give you gas, and most people adjust to that after a week or so. Never hurts to try them at any time.