It absolutely positively does not let you lower your FC level. Unlike ozone, the UV doesn't kill algae floating through it very much (they are heartier than bacteria). Also, like ozone, UV only kills pathogens that get circulated to it and do nothing for bacteria or algae growing on pool surfaces or that otherwise does not get circulated. So you cannot have a lower FC level in the bulk water if the purpose of that FC is to prevent algae growth and to kill pathogens, especially preventing person-to-person transmission of disease (i.e. for pathogens that don't have time to get circulated between going from person A to person B). This latter argument is more relevant for commercial/public pools -- the risks in low-bather load residential pools is far lower.
Finally, the UV is a mixed bag since it destroys some disinfection by-products but creates others. It's fine for indoor pools where there is no sunlight so something is needed, but for outdoor residential pools exposed to sunlight there is much more UV from sunlight than from the lamps though the spectrum is different (the UV lamps have more germicidal UV). Also, if you have a very powerful UV system you'll actually use MORE chlorine since the UV will break down the chlorine (same is true for ozone).
