If Uckfield is not electrified, then you need class 755 bi mode trains, but with batteries instead of a diesel engine that can work on DC power and that has regenerative braking where, the power in the braking on DC line gets inserted back into the battery packs as well as those packs being charged as the train runs. At the Uckfield end, there would also have to be a point that a train can be recharged from local mains if required.
But since there has been issues with the class 777 units, I doubt that the above will happen.