Official answer is 5500 lbs in High Range, 7700 lbs in low range. Anything over 1250 lbs must have trailer brakes. I believe in Europe, there is no 5500/7700 differentiation, and they state the weight at a flat 7700 lbs.
We all know these vehicles are very strong. You certainly are capable of towing larger loads, but the real limit is your stopping ability. Also, if the heavier your load becomes, the more unsafe driving with it becomes.
We have towed MANY rovers behind our Discovery, and even our Defender. It is slow and takes 8 years to stop. Towing something with a longer wheelbase isn't very fun. We flat tow Noah behind us, and because he has such a short wheelbase, he just follows along & it's a piece of cake. The other issue you'll run into with towing your Discovery is that it really should be trailered, and that trailer adds considerable weight to the picture.