It is possible that the new Iridiums on the A27 in West Sussex use a new type of lamp:
Philips CDM-T Elite MWThese new lamps are being marketed as an extension to the range of CDM-T and CosmoPolis lamps, bringing higher efficiency white light to higher wattage applications. The MW suffix stands for medium wattage, as the lamps come in two wattages, 210w and 315w. They are said to be for both indoor and outdoor use, and my attention to their existence was raised by looking at the Urbis Evolo PDF. The warm white 210w lamp produces 24200 lumens and the 315w lamp produces 37800 lumens. The lamps themselves are similar to CosmoPolis lamps, but slightly larger, although much smaller than CDO-TT/SON-T lamps. They have a PGZ18 lampholder, which is like a large version of the PGZ12 lampholder used for CosmoPolis lamps. The control gear for the lamps is electronic like CosmoPolis is required to be. These lamps are approximately as efficient as standard Philips 250w and 400w SON-T lamps, although due to their slightly reduced wattages produce fewer lumens of light. Of course, they may produce slightly fewer lumens of light, but the light is white rather than orange, so they are unlikely to appear dimmer and would probably seem brighter.
I was unsure of the lamp type used in the A27 Iridiums, as I mentioned a few posts ago. The lamps seemed too small to be CDO-TT lamps, but the lampholder appeared to be porcelain rather than the metal of CosmoPolis lamps, and the light output seemed too bright to be 140w Cosmo anyway, but the lampholder didn't look right for a CDM-T lamp with a push in G12 lampholder. These new CDM-T Elite MW lamps seem to fit the description, and if Urbis offers them in the Evolo then Philips is likely to offer them in the Iridium. I am guessing the ones on the A27 are 210w lamps, as when I saw them at night I likened them to 250w metal halide.