There is no failure to stop here, you commit an offence if you impeded traffic flow, so you used your common sensem unlike the copper, who could have signalled you just before the filling station.
If you have NOT been warned about the illegal plate before, you have to be given a warning first, and then you can either fix it or ignore it as you wish, but first one is a warning always, it WILL be put on the system that you have been warned so next one is a fine, then next one is withdrawal of number, the take the plates off there and then.
They then either let you or arrange themsleves recovery of a "unregistered" vehicle to the pound.
They will not and cannot let you drive it away with no plates on.
tabetha