The officer was responding to an emergency that had been resolved before your conversation, so the van was necessarily parked in a disabled bay. But, as you clearly state, this is not the real issue here. The officer was not justified in asking you to delete the photo and it's right for you to draw attention to this.
Interacting with private photographers or the media is not something police officers do on a regular basis, which means mistakes are sometimes made. In this case, the officer was quite new to the force and had not come across such a situation before. That said, you are not the first person to raise this and it is something Surrey Police has already started addressing.
We provide media awareness training to most front-line officers. In the future these sessions will make clearer the wide rights photographers have to record police activity, provided cordons are not crossed. We will also soon be distributing a short booklet of media advice, which includes information on photographers' rights.
Thank-you for responding politely and explaining you objections logically to the officer. We would advise others who find themselves in a similar situation to do the same. It's also good to note that the situation was resolved using common-sense after discussion with the supervisor.