Gilbetus's channel

It means the bonnet itself (of his Rolls Royce car), not the bonnet ornament (i.e. the 'silver lady'). They're putting some sort of blacking make-up his face and the valet wants to polish it! Sir Henry is saying that he doesn't want to look like 'the bonnet of me Rolls', i.e. shiny, as Rolls Royc...