as a brit who has yet to go to the USA is there any major difference in price between
walmart, target, Publix & Goodings?
It depends on what you're looking for. If you need things beyond groceries, then Walmart will be the cheapest and most convenient to WDW, I believe. The closest Target is going to be all the way down and around on 192, which is a pain in the traffic any time of the day.
However.
People will try to tell you that WalMart is cheaper for groceries than Publix, but as someone who's shopped at Publix my entire life, I can easily tell you that the prices, over all are going to be spot on to WalMart. For some essentials, like milk, the prices at Publix can actually be cheaper.
Here's the other thing... Publix isn't nearly as scary as WalMart. As a Brit, well, um... so you know, sometimes going to WalMart is like going to another world!! (and not in the good way!

)
One more thing in Publix's favor, they tend to offer a lot of imported items, so you might find a piece of home there, too!
Goodings is not worth your time or money. The closeness will not save you the hassle of spending so much more there!