Every 'active' TV channel (i.e. one that you're either watching or recording) requires a tuner unit to convert the signal from your aerial to sound and pictures. When you use an external device (such as an old-fashioned VHS recorder, a DVD recorder or a PVR) to record one programme, while watching another, that's possible because there's one tuner unit (inside your telly) providing the signal for the programme you want to watch, while there's another tuner unit (inside the recording device) providing the signal for the programme that you want to record.
Most (all?) TVs that have a built-in recording function only have ONE tuner unit inside them, meaning that you can EITHER use it to watch a particular channel OR to record a channel. However (because there's just the one tuner present), you CAN'T do both things at once.
So, despite the fact that your TV has a recording function built into it, the only way that you'll be able to record one programme while watching another is to buy a separate recording device (such as a PVR). The most popular make, by far, is Humax
https://www.argos.co.uk/product/7865451?clickPR=plp:1:2