BA European departures are separate from most of the action at LGW, if I remember correctly--definitely a separate terminal from CO & VS.

My memory of the terminal set-up (fuzzy) is that you might indeed need to clear UK entry to transition between terminals, though there may be a 'back door' route I never noticed.

If you're at LGW for the layover, you could take the Gatwick Express in to Victoria Station. It is 30-40 min each way (~10 pounds), but Victoria is busy & central enough to make killing a few hours very pleasant. London's first EasyEverything internet store is also just across the street (lots of computers, cheap internet access)....