Hang on a minute, you could try changing your TV setting (not the box setting) for them channels, but to be honest uk gold broadcast some older programmes, which would have been shot in the 4x3 aspect ratio, before 14x9 and 16x9 formats so all you would be doing is stretching the image and distorting it to fit the full screen, does the aspect ratio change when the adverts come on them channels? TBH I dont think its the box just the format they are shot in.
But try changing the TVs aspect ratio to 16x9 instead, it might be set to auto at the moment which is why its switching for the 4x3 formats.