You don't need an external aerial for this. The STB decodes the digital signal and outputs it on both the scarts and, in a format similar to an ordinary terrestrial analogue signal, on the RF output. All you need to do is connect a TV (or and TV card in a PC) to it and away you go. The TV (or card) stays tuned to the same channel all the time - you have to switch between channels on your STB.
If you had an external aerial then you would be able to combine the signal from this with the output of the STB so that you could tune into any of the terrestrial channels OR the output of the STB. This allows you to watch different channels on the main and remote sets - provided that only one channel is not free-to-air.
If you have one of the old STBs with something called an analogue pass-through then you don't even need an external aerial. All the signals are supplied down the same NTL cable.
There are some pictures and word of how to connect everything
here.