My dog doesn't eat unless he has permission. So I guess that's where I entered on this. When I'm in the final stages of meal preparation, say, last ten minutes, I put his food down and have him sit next to it. When we sit down to eat I say "OK!" and that releases him to eat.
On a one or two occasions I went to eat outside and forgot to release him. I came back in half an hour later and he had lain down, food uneaten. There was a puddle of drool. Poor guy. When he is staying with other people he waits for permission, too. While staying with another dog he was quite dismayed when, sitting quietly by his bowl, the other dog (not trained to wait) went and helped himself.
My hunch is that I established very early that his access to food depended entirely on my say-so and that this has carried over to other things. But it might also be his personality. He's extremely non-aggressive. In fact I hardly ever hear him bark. Maybe once every month or two.
It could be the novel surroundings, the fact that he had "discovered" a new toy/treat on his own outside the home, or the fact that other dogs are near. I would work on the trade game in various outdoor surroundings with no other dogs around. The only time Jack has every snarled at me is when he had found or been given a highly coveted item and another dog was trying to steal it, and I tried to take it from him. I actually think the snarling was triggered by the other dog, because otherwise you can take things right out of his mouth.
Frequently, saying "leave it" before the dog gets it is better in theses circumstances. If that is not possible, try walking with him to a quiet area where there is no one else around, then trading for the item.
Beth's advise is excellent. Learning to "leave it" could save his life some day. It's one of the most important commands they need to learn.
Maybe you could try teaching 'Drop' instead.
I like this trainer's technique.
I agree with Beth's suggestion and it sounds as if you are working on that. Corgis were bred to keep critters out of the garden and tell one ton animals what to do. They often do not react in a mild mannered way to things they don't like. Some are easy but some will resist as Reginald did especially as an adolescent. This is an age where they will try their parameters a bit. Work on the trades but when you are removing something it does help to say "leave it" and then while crowding his space a little wait for him to drop it before completing the trade. Cesar demonstrates this on his show often, That pause helps remove the instinctive "it is mine" response. With training this stage should pass :) Corgis can be very strong willed characters!