As best I can tell, PG Tips (which I still find to be a funny name for a tea company) first used chimpanzees, ala Lancelot Link, in commercials back in the 70's. Later, they developed a sock monkey to be their mascot.
Search YouTube for "PG Tips Monkey", and you'll find some of his commercials. Thanks in part to a legal dispute, he apparently went on hiatus. This clip, is appropriately named, "The Return".
In typical British fashion (no pun intended), he's even been known to don a dress and wig.