Towing a larger trailer with the LR3 seems to raise the temperature a bit, I have seen many threads on the subject, have look over at airforums.com
I have a 2015 25ft and it is too much trailer at 7600lbs and approx 800lbs tongue.
I don't recall the 23D numbers, but I seem to recall the tongue weight was the major issue, not the total weight.
With regards to the standard hitch, for safety, always go with the lowest rated number, in the manual it states 550lbs, the hitch is stamped with 770lbs
Remember, Land Rover do not recommend the use of a WD hitch, that is in the manual.
People also question the short wheelbase of the LR3, saying for longer trailers it could reduce stability, I do not really think there is much concern here as the LR3 weighs so much.
Please contact CanAm
http://www.canamrv.ca as they are probably the most experienced company for modifying cars/trucks to increase the ability to tow. The advice you can get from them can then be used locally to get any modifications done you may need. It is very common for them to recommend certain hitch reinforcement to the chassis.
Please pay attention to the GAWR (gross axel weight rating) and the GVWR (Gross Vehicle Weight Rating). These are on the door sticker, take the time, do some sums, ask for help on the forums if you are getting a little unstuck...
Finally, the LR3 is an amazing tow vehicle. No doubt it can tow more then the paper spec says, but safety has to be the number one priority, don't risk your life or those around you, get professional advice
I hope this helps.