My second prototype, this time from The Gamecrafter, has arrived. Here it is next to the one I ordered from The Game Maker:
(The Gamecrafter pack is the one on the left in case you can't tell the difference). It's just what I expected and hoped for, and is a professional looking pack (in my humble opinion). It's also pretty much exactly the same as the prototype from the UK supplier; if one didn't have "US Edition" written on it, I wouldn't be able to tell them apart myself. However, this can only be a good thing.
All I have to do now before I can start a Kickstarter is calculate how much the packs will cost when bought in bulk, and how much postage is going to cost to various destinations around the world. It looks like The GameCrafter do a kind of bulk-order fullfillment which should help a bit, but I have a feeling I'll be packing UK orders myself.
On a similar note, I had a great idea today: just like 3D printers have been invented, the world could really do with a proper commoditized "card printer". Customers could download "card templates" and the printer could print out professional-looking packs of cards. It would save so much of the problems of posting out packs of cards to everyone.