It's under rated for tongue weight capacity I think. I just towed 750 miles round trip with about 8000 pounds. I am sure I had at least 700-800# of tongue weight. It was a dream to tow and extremely stable.
I think the tongue weight rating is a factor of GVW and having to keep the vehicle within a specific class in the UK. If they rated it for more than 7700lb towing or 550# tongue weight it would be in commercial/heavy vehicle territory.
Having installed the trailer hitch assembly on this vehicle myself I can assure you it is extremely heavy duty. The L322 setup is vastly superior to the LR3/Sport "quick hitch" setup.
I'm not an engineer, but based on the hitch design and construction itself, the weight of the tow vehicle (well over 6k with driver, fuel, gear) , load leveling air suspension, size of the brakes, I think it would be just fine for loads approaching 10k (properly loaded trailer).
Just about the only drawback is the wheelbase at 113 inches, but that's not too short for safe/stable towing IMO. Most of the regular cab short bed half ton trucks on the market today are about 120" wheelbase and are rated for 9k plus.