The data you're referring to is the power type that the timings are based on, nothing to do with the motive power that will run the service. For example, the Gospel Oak - Barking services are all timed for a "Class 150/153/155/156 DMU", yet are run using Class 172 DMUs.
I am working on getting a reliable source of formation data - it's tricker than you might think... but it is something a lot of people will benefit from!
If you look under traction history, each day on the calender has a list of recorded traction of some services. Obviously, because of different factors, the formation/unit type isn't always going to be 100% guaranteed. Hope this is of some help.