If your live NowTV Broadband activation has already gone live and you still haven't received the NowTV Hub Router yet, then yes you should be able to use one of the Sky Routers which should work depending on the Sky Router model number.
Once the NowTV Hub Router arrives and it's the day of your NowTV live activation date, then use the NowTV Hub Router initially.
Should you find the wifi coverage is poor or you need more than two Ethernet ports on the Router, then maybe try the Sky Routers instead.
I don't know if the NowTV Broadband Buddy feature works with the Sky Router, because i haven't got a Sky Router to test this out.
Should you need to contact the NowTV Broadband Team or do a online Technical Check, then make sure the NowTV Hub Router is plugged in before hand.