it is all down to what kerb markings there were.
Loading and unloading is allowed on double yellows as long as there are no yellow lines on the edge of the kerb. This has already been posted earlier.
You were clearly unloading - therefore you need to ‘clause’ the ticket and provide evidence of this. You should also request that the particular warden is retrained.
Clunk:
I once delivered to this place.
The loading bay had cars parked in them, so I stopped in the road and started off loading 8 pallets. As I opened the doors, I started unfolding the tail lift and a car stopped too close to me to allow the tail lift to unfold. I asked him to drop back and he refused. So I pull forward and went back round the back and he was less than 6 inches away from my tail lift. I asked him to move to which he replied ‘go around again I need to be somewhere’.
So I said fine, closed up the back doors, and opened the side. he came over and asked what I was doing. I said that as he wont move back, I will handball the boxes into the shop instead. He started shouting at me then after a while got back in his car constantly beeping his horn. I just ignored him.
The cars started piling up behind him, and some started honking.
After 5 mins or so, a copper appeared. Asked what I was doing. Explained that I had 8 pallets to unload, and the loading bays were blocked. And about the guy behind blocking my tail lift deliberately thus slowing me down.
He grinned at me, and said ‘take your time’.
He then got his book out, and started writing tickets for the three cars in the loading bay. As he was doing so, the car behind me honked his horn. The copper then wrote him a ticket for disturbing the peace.
I was there 40 minutes in the end. With the copper nearby dealing with the complaints.
the coppers hate those prats even more than we do. i had a bmw blocking me in while i was delivering to a co op in london. i was on the pavement as there were car on both sides of the road as far as the eye could see. came out, bmw parked on the corner where i accessed the pavement and there are chevrons there to indicate no parking. called the police and a scottish woman came with a rookie. i explained the problem and she wrote a parking ticket and a note to tell the driver off. then she decided she had more to say and wrote another note. while we were waiting for a recovery truck to lift the car a few streets away she explained what they were doing. they lift the car and rather than impound it, the drop it a few streets away so the owner reports it stolen. they invte them to the station to fill in a report so it saves them trying to find the owner. unfortunately someone stuck their nose in and told her where the owner worked so she went and got him. i dont know what was said but i have never seen anyone hang their head like that. she must have torn a big strip off him 
used to do multidrop for a well known and now defunct wine merchant. used to get parking tickets reasonably often, and the office would challenge every one on our behalf, so long as it was with them within 24 hrs. never had to pay one yet. they used to just get thrown out of challenged, with a covering letter along the lines of “we’ll let you off this once, but don’t let us catch you again…” we suspected that they just didn’t see it as worth the hassle for trying to get their £30 out of us
The point i was trying to make was the fact that it was a small van, and if i didnt park 2 wheels on the pavement, i would have blocked the road completely, so i was trying to be polite, and show what idiots parking wardens really are, problem is, you cannot reason with them.
On another instance i was on tippers in the city of London, the site was a one in…one out job, and after loading each driver would try and remove as much wood as he could, then it would go as ■■■■■■■■, anyway it was in a 4 lane road, i was waiting in a tidy spot, no obstruction, and waiting for the other driver to pull out, when up comes this coloured chappie, dressed as a warden, so after him telling me that i would get a ticket for illegal parking, i told him what i thought about his colour, but with a swearword attached, and a reference to a person born in sin ( so to speak )…he was having none of it, neither was i, and ended up getting a ticket anyway, but shows the lack of brains.