Your windowsill is separate to your uPVC windows therefore they are not included in your uPVC windows.  The windowsill is part of the buildings structure and is necessary for keeping the window in place and improving the windows energy efficiency.

Your new windows will be manufactured to fit the existing aperture and be fastened directly to the outer walls with a small amount of sealant put in place to prevent draughts.  The UPVC units do include the exterior window ledge as part of their construction, however the interior sill is not part of the system.

If you do want to replace your windowsills to improve the interior look of your home, then doing it at the same time as the windows is possible as the removal of your old windows will allow easier access to the sill for replacement.