If you get a strong enough TV signal where you live, receiving Freeview on an internal indoor aerial is perfectly possible, but for better results, an external aerial pointing in the direction of your local Digital TV transmitter is a much better bet. Indoor aerials are less able to pull in signals, and more susceptible to interference from domestic equipment (computers, washing machines, etc). It's often hit and miss with indoor aerials, and as the transmitters for digital TV operate on a lower power than normal analogue TV transmissions (so as not to interfere with analogue), you're less likely to get a good digital TV signal from an indoor aerial. As a general rule of thumb, if you get a poor analogue TV signal from an indoor aerial, you don't stand much of a chance with digital. Entering your postcode into Wolfbane's coverage predictor gives you an aerial recommendation that can help to give an idea of the effectiveness of an indoor aerial.
http://www.wolfbane.com/articles/ukdmap.htm
Click on the transmitter nearest you.