It seems to me that the table-service restaurants always have at least one sugar-free or low-fat option, but it's generally something like dry grilled chicken or a salad. If that's ok, then there's no reason to make a request. However, if you're going to want something different, you may need to request ahead of time. Some restaurants will accomodate special requests once you're there; some will only make accomodations for religious reasons or allergies.
I had a great experience a few years ago at the Crystal Palace in the MK- my cousin keeps kosher, and my father is on a low-carb diet. When we mentioned this to our waiter, he brought out the chef, who offered to make my cousin a special dish in her own seperate cookware, and personally took my father on a tour of the buffet, pointing out what he could and could not eat. So, I highly recommend CP if you're on a special diet.