The sole purpose of knowing where the jackshaft is at is to make sure the distributor rotor points to the same direction as it did before when you reassemble. You can either bolt something across the jackshaft sprocket or you can pull the distributor cap off an just make a note of where the rotor points to. Jaguar made a special tool for the former, but the latter is just as easy a method.
Similarly, it makes no difference what order you tackle the heads in. The objective is to put the camshafts back on exactly the same way as they came off. If you fail badly, the pistons will smack into the valves, so just pay attention to where the cam notch is and will be fine so long as you don’t remove the circlip from the cam sprockets and inadvertantly separate the the two halves of the cam sprocket from each other. It’s quite hard to be 90’, 180’ or 270’ out.
kind regards
Marek