Whether you need a TV licence to watch NOW depends on what devices you have in your home.
I have a TV in my home
If you have a TV in your home, you'll need to buy a TV licence, even if you only stream NOW using other devices.
Government guidelines state that:
"Once you are in possession of a television set capable of receiving television broadcasts, you are required to have a Television Licence. Effectively, it is the set that is licensed, not the ability to receive television broadcasts."
I don’t have a TV in my home
If you don't have a TV in your home, and you only stream NOW using other devices (such as a computer, tablet or games console) that don't contain a TV tuner card, then you won't need a TV licence.
Government guidelines state that:
"So long as the computer is unable to display television channels distributed by conventional television broadcasting networks (i.e. cable, satellite, IPTV, analogue terrestrial, digital terrestrial or MMDS) e.g. using a television tuner card or similar device, then there is no requirement to hold a Television Licence."