If it had SAI before and it was removed, the holes in the heads would have to be plugged, the SAI apparatus removed, and the ECU reprogrammed or replaced. I just checked an Atlantic British video and there is not a VIN range or specific model that had SAI or not. On my replacement engine, it did have SAI so I removed it because my Disco didn't have it originally. I just plugged the holes. So I'm guessing your Disco didn't have SAI to begin with. Be grateful... Unnecessary complication and expense.

Your codes are probably related to some vacuum leak somewhere. It is raining or I would take a picture of the top left (facing the engine) rear line on my upper intake. It is cut short and has a bolt in it with a clamp on that. I basically plugged it off and put a breather filter on the valve cover. That's where that kine goes and there is one on the other side, rear of the throttle body, that does the same. Be sure and check those lines for seal and any cracks. The one right in front of it is a hard line that boosts the brakes. Just below both should be a cap over a port that isn't used.

The leaks I have found were at those locations, the injector o rings, and the gasket between the upper and lower intakes. I think I read that you smoke tested it and found nothing. Did you pay particular attention to any smoke escaping under the upper intake? If so and you are positive that you don't have any leaks, I'd also look for an exhaust leak prior to the front O2s. You should be able to hear that during a cold start. Then, finally, the MAF. It can do crazy things if it is failing.

I hate to suggest all of these things because I am pretty disgusted with my engine right now, BUT for the faults I have now (misfires due to valves, probably) and having done all of those things, I got rid of my own longstanding lean/rich codes.