It depends so much on the signal in your area and the line of sight from your house, Milton Keynes for example has quite poor coverage being halfway between two transmitters (and relying on its silly cable solution) and prior to the switchover we had poor analogue signal and almost no digital signal, since the switchover they boosted the transmitter strengths but its still quite poor. Those internal aerials very rarely work here, tried a few of them and they just about worked outside last time i tried, pointed directly at the compass bearing of the transmitter but not inside even a window/wall reduced the signal too much.
This website is awesome for telling you what signal strength you should get and what bearing the transmitters are on if you enter your postcode although its a little on the pesimistic side
http://www.wolfbane.com/cgi-bin/tvd.exe?DX=D&HT=10&OS= You can see if you look at the columns under each "ERP W" that the commercial multiplexes tend to transmit at a lower signal strength so if youre borderline you can sometimes get some groups of channels not others. A list of which channels are on each
http://www.ukfree.tv/allchannelsmuxes.php
Im no expert btw, I just ended up geeking it a bit to try and get a signal pre-switchover