Yes you can.
(i) If you have a Membership of Now, whether direct from Now, or via TalkTalk, or via BT, you can use it on any device that supports Now, as well as (or even instead of!) your YouView box.
(ii) If you have a TV that doesn’t support the Now app, then you can use it with a Now Stick, or an Amazon Firestick, or a Roku device plugged into an HDMI input on that TV.
Or indeed any device that supports Now, and plugs into an HDMI input on your TV, of which a YouView box is just one example.
You may also find it useful that you can get subtitles, where Now provides them on a programme, on most other devices than a YouView box, which doesn’t support subtitles.