Convoys using IETs (Class 800/802 EMUs) can be started in Electric Depots, but not completed:
(https://i.imgur.com/4Dl060a.png)
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.
Looking at this again, it should not be possible to build the 800/802 in the electric depot at all, as all the vehicles are marked as diesel, but this does not appear to work for the front vehicle for some reason.
You can build the 801 in the electric depot, however.