A Nip is Notice of Intention to Prosecute, ie. it's you being informed that you're being reported for a motoring offence & as such it can be written or verbal. The written one is typically sent when you've not been stopped & talked to by plod at the time of the alleged offence eg. getting flashed by a speed camera, whereas if you got pulled by a live copper & he didn't offer you a fixed penalty option, & told you that you were being reported for speeding then that is the NIP delivered.
So, from what you say happened, the Traffic Warden has delivered the NIP already.
Of course the above applies to coppers & 'proper' motoring offences, I've no idea if Traffic Wardens have the same rights & restrictions as the police & as mentioned in the previous post there are also different types of wardens & I doubt they all have the same level of authority.
Probably just another little Hitler gobbing off because you've disrespected his overblown sense of power, I wouldn't worry about it to be honest.