In order to electrify any of them, you do need a solution to the overhead gantry loading problem. Be that wholescale replacement of 66s with 88s/93s/99s, battery tenders for 90s/92s, on-site shunters, moveable conductors or some other solution.
You also need sufficient electrification to get a decent workable diagram for electric freight locos, and while that's /getting/ better, it's still far from the network coverage it should really be for that work.
FWIW, I would say London Gateway absolutely should be electrified as far as is necessary for freights to stand clear of the main line while switching between the pantograph and whatever auxiliary power they use. Bi-modes would do it for the moment and we'd have to just accept diesels continuing for now, but the long-term goal would be that all electric locos have batteries suitable for shunting, making the top-loading solution as simple as possible. Loco pulls up to the docks, drops the pan, uses the battery to get to the right position for unloading, is released and runs up to a charging point. On departure, it drops on to the train, couples up, uses the batteries to get up to the mainline protecting signal, pans up and then driver calls signaller to report ready for the road.