I've no experience specifically of Now TV boxes but I would guess it is going to behave pretty much like other Internet based UK TV services. Firstly it will use your IP address to identify that you are located in Greece and then it will refuse to stream content because they only have a licence to stream it in the UK. It might be possible to use a UK based VPN to fool it into thinking that you are in the UK but I suspect there will be no mechanism to allow you to turn a VPN on in the box the box. You will need to be able to get your router to turn on the VPN service. I tried to do this with my router but had no success.
I would guess you would be better off getting a more general purpose Android TV box and loading Kodi onto it. See this thread
for a lot of discussion.
Alternatively, if you are prepared to pay a monthly fee to get a wider choice you could try Ex-Pat Vision