To switch to Paypal payment you need to be an existing customer according to this linked help article below.
With regards to using your bank card to create a NowTV account, the card needs to have either the Visa or MasterCard logo on it, the card needs to be from a UK registered bank where you are a UK resident 18 years old or over and have sufficient funds in your account.
I have come across people who have attempted to use up front pre paid credit cards setting up a new account encountering difficulties with the nowtv online system.
See if NowTV live chat can assist you further.
Open the link page below and select the most appropriate help topic, then once you get to step 3, scroll down to the bottom of your screen page, where hopefully you will see Get in Touch (click on the drop down arrow below it to reveal the live chat button).