Don’t forget, Keith is a character played by the very normal-looking Leigh Francis. But Leigh has become Keith at this point. You can’t separate them. When Leigh goes to a party, he probably ends up doing the Keith voice by accident.
Remember Bo’ Selecta!? That show was a fever dream. Keith started as a mad fan, and now he’s a national treasure. A weird, slightly rude national treasure, but a treasure nonetheless.