It is possible that the new Iridiums on the A27 in West Sussex use a new type of lamp:
Philips CDM-T Elite MW
These 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 have a PGZ18 lampholder, which is like a large version of the PGZ12 lampholder used for CosmoPolis lamps.
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.Well a trip along there today on the way back from a work trip and a handy layby produced this:
So yes they do look like the new Elite MW MH lamps.
It looks like the porcelain base of the lamp fits inside that metal collar which must be the lampholder.