I fit the odd aerial and the best bet is to fit an outdoor "Digital" aerial as the signal received by the box is better. The aerial doesn't have to go on the roof any outside wall will be ok as long as it's pointing in the direction of your local transmitter. A digital aerial is not expensive and most B&Q do them for less than �20.
Also don't get conned into getting the ones that have the yellow or blue bits of plastic on them that cost about �75 they do the same job as the �20 ones....