There was a high speed express tank: The 'Remembrance' class 4-6-4 'Baltic' (The name given to 4-6-4s) owned by the LB&SCR. (London, Brighton and South Coast Railway)
I don't think Ivatts worked on the Metropolitan, except if the Titifeld-Mallingford line has one! They did, however, have 0-4-4s that had condensers to limit the amount of lost water in tunnels.