You've not stated the type of recording device which you're hoping to use.
In the 'old days', when you used a video cassette recorder with an analogue TV, you could record one channel while watching another because both devices had their own analogue tuner. (The tuner in the TV fed one signal to the screen, while the tuner in the VCR fed another signal to its tape).
To get back to the same position with digital TV you again require two (digital) tuners. i.e. you can only record one channel, while watching another, if your recording device has its own digital (Freeview) tuner. If your recording device doesn't have any built-in tuner (like a few early DVD recorders) you can only record the channel which you're watching. If your recording device only has an analogue tuner (like VCRs and many pre-digital DVD recorders), you'll be able to record one of the five analogue channels (until the transmitters are turned off) while watching Freeview.
Chris