Ae you saying an ounce of coins costs £80 more than an ounce of gold bar?
That would be expected, as you cannot buy one ounce of gold bar but only a kilo - smaller quantities always carry a premium. Also, the coins don't mint themselves, and the dealer network also needs to be financed. Obviously, there is also an element of demand and supply in that premium, but not sure it reacts much to external events. Is there a chart where you can track the premium? A quick google returns
this, no development over time but at least an explanation of the various premiums for different coins.