Convoys using IETs (Class 800/802 EMUs) can be started in Electric Depots, but not completed:
They can be completed in Diesel and Modern Traction Depots.
The BR-800Front vehicle is marked engine_type=diesel (https://github.com/jamespetts/simutrans-pak128.britain/blob/584d17bfa5851b6da46167832fbae0cbb3f87f47/trains/br-800.dat#L9). I realize that the game engine does not yet properly support bimode vehicles, but the current arrangement just looks broken if you open an Electric Depot expecting to find IETs.
I don't think this is related to the 'Show all' bug in the current Main (Master) branch, as I tested this in the depot-dialogue branch using Ranran's new UI.
I believe that, until bimode vehicles can be supported, the class 800 is intended to represent diesel powered units and the class 801 electric powered units.