Topic

Can anyone please identify what the cable connects to?

If that is the front wiring loom, I suspect that it is the other earth connection to the power steering. I'm guessing that the object at eight o clock with the two screws is the 40Amp fuse in which case, this might help - just a guess.
https://www.mgfcar.de/schedules/sb14.jpg

If you mean the thick brown wire with a ring terminal, then it won't be an earth wire, earth wires will be black.
Brown is for unfused power, so probably either the starter or alternator.

Highly likely. It would help to know which loom we are looking at.

Looking at the amount of sensor connectors, it must be the engine wiring loom

Sundance

Yes, I can see the other ECU plug, now. I think it will be for the altimeter or the spin cycle timer. It might as well be. Why? Because in sixty years of reading (and drawing) circuit diagrams, the MG ones are the clearest I have ever read and by a long chalk the least comprehensible. They make Citroen and Mercedes diagrams look like Janet and John! Just leave it out. If it doesn't start, it is the starter, and if it doesn't charge, it will be the alternator

I have just completed extracting from an MGTF engine loom exactly what you are looking at to retrofit to my MGF. There is a brown wire going to the starter (ring connector)- the other end goes to the 80 amp fuse held in a translucent box (left side of your picture), the other end of which goes to the alternator in the form of a square connector, which I believe is what you are asking about (all wires are brown and very thick). Basically, a brown cable connecting starter and alternator via the 80 amp fuse,

Looking at its distance from the main ECU plug I would say it's the main power connection for the starter.