I know what you mean by overscan - I had to do something with that (but on a different TV) to get a perfect desktop fit with a laptop using HDMi port But this is 'cutting off" a lot more than that. If you're familiar with the "BBC" watermark in screen top left, all you can see is the "C".
I can't test either as this is a really old Plasma 42" (Panasonic TH-42PV30) and Setup menu is very basic with no advanced setting and nothing for 'overscan'.
Ok on that slider being volume. Guess I'll leave as is.