Holby is a fictional city near Bristol, just as "The Archers "and Borsettshire is in the Midlands. I recall from the first episode it showed Charlie driving over the Clifton suspension bridge. I'm convinced it used to be called Holby General, and was surupticiously renamed Holby City so that when the spin off call Holby City was launched they could spell it "Holby Ci+y" in the same way they spell out "Casual+y"
It is filmed in a large warehouse in Bristol, whereas Holby City is filmed in a disused office block next to Elstree studios in London.