Whether working out a heavy T. Rex obsession on ''Seen the Light'' or doing an XTC-smashes-the-Who's-guitars number like ''Never Done Nothing Like That Before,'' this London quartet continues to evince range and spunk aplenty on Life on Other Planets. They're uneven, to be sure. Most original rocker: the thrillingly terse ''Grace.'' Biggest load of bollocks: ''Prophet 15.'' But Supergrass are never less than lovable.