As long as you plug a TV receiving/tuning device into them - from a Freesat/Freeview set-top box to Sky, Virgin etc boxes, or a TV-tuner-equipped PC - you can watch TV via your projector.
You can also run TV sources into an AV receiver, which will upscale them (and other sources - such as your DVD player) for output via a single cable to your projector.ÿ
I'd urge a TV source and/or AV receiver with an HDMI-output for the best quality - any loss in picture quality is incredibly obvious on a huge projector screen!ÿ