If by "off the shelf" you mean stock photography and/or illustrations, then the answer to whether or not this is permitted depends on the company from which you license the art. When you pay for art on a stock art web site you are not buying the art in question. Rather, you are paying for a limited use license for that art. For example at iStock, which is now owned by Getty, a regular license will NOT allow you to use the art for items for resale (this would include chocolate bars). You would need to purchase an extended license for that.
Many stock sites have similar arrangements, and you'll want to be very careful to be aware of those before you move forward. BUT, once you insure that you're meeting their terms, then you should be fine. Most of these sites (at least the good ones) guarantee that their art will not put you in a position to infringe.
On the other hand, if by "off the shelf" you are referring to those cheapie web sites that create logos and such for $50, then I cannot advise strongly enough against using them for this type of situation. Really, just don't.
Also, no matter what advice you receive here, I cannot recommend strongly enough that you run this past your attorney. I don't know what infringement penalties are like in the UK, but here in the US they can be so draconian that even a small transgression can put you out of business. You'll probably be fine. But proceed with caution.
Okay this is different. I didn't realize that you were talking about decorative and stationary paper. In that case I would simply call the manufacturer and ask them. They'll probably be fine with it. In fact they may really like the idea that you're using their paper for this purpose.
In addition there's a good chance you'll be able to buy wholesale quantities direct from them at a substantial discount. They may even be willing, as long as you buy sufficient quantities, to produce paper for you in a custom format. With the right manufacturer, you could develop a very beneficial relationship that you could even use with your new designs!
You might want to verify that your packaging complies with EU and UK legislation concerning food safety. Any (packaging) material in contact with food is subject to these regulations.