You left her on a road. Out of sight. For an extended period of time.
I'd report you. Mainly because I would wonder where you were & how long you would be. I don't know you, so wouldn't feel best placed to assess the case. In my training I have to stay child focused, the child was distressed. I don't know if this is a regular thing for you.
A sandwich isn't really worthy of leaving a toddler in a car for 7 minutes. Neither is a coffee, to be fair, imo.
Your 'the worse' scenario = her crying for 5 minutes. It is quite easy for a toddler to be distressed in those 5 minutes, she has no idea where you are, if you are coming back. She was probably scared by people peering in on her..
Can I still get reported to the police? Based on my number plate?? Well yes, of course you could. Either by the ladies, or the SS if they think it warrants a visit. The fact that you feel so bad tells you all you need to know, that you did wrong. You extended your "1 minute for a coffee" to 7 minutes for a sandwich.
You have started on that slippery slope. And you need to pull it back.
People take their children into the petrol station to pay? Really?Why? I can hand on Heart say I've never seen this! You have to here (Australia) and not just because of the heat. Parents have been prosecuted for leaving kids in the car to pay for petrol.
You parked in front of a police station ffs. Sorry, must have missed the memo, does that make 100% sure that nothing will ever happen? Did she ask the police to babysit for her? 
She had taken the trouble to park somewhere sensible. On the side of the road?
Spring she is a parent - she needed to do more than take the trouble to think about where she parked. Remember she wasn't on an emergency mission but a trip to get a non essential sarnie Agree! There was no need to leave her daughter in those circumstances. When you are a parent there are things that are a bit inconvenient. Going out in the rain with a toddler, heaving a seat/baby out of the car. It is all part of creating life & shouldn't been as an inconvenience. For the first 5yrs of my eldest life, I didn't drive. That meant walking every where in all weather. The great thing about raising children these days is the fact that we have baby equipment that can stand all weather. We have rain guards etc.
I am sure that if your daughter was hungry & you had no food in, that there was a shop closer/within walking distance that you could have got food from. You made a totally unnecessary journey op & your leaving your dd was also totally unnecessary.
And yes, I do leave my children in the car to pop into a shop - but I can see them at all times. No way would I have left my child, alone, out of sight for 7 minutes. Car or no car.