You missed the fact that a 1200kg saff on a 500kg trailer weighs more than the kerbweight of the car, so you would not be allowed to tow it without +E

I also believe the 3500kg limit applies to the gross weights of the car and trailer, not the actual weight.

Gross weight of towcar + Gross weight of trailer must be below 3500kg
and
loaded weight of trailer must be less than the kerbweight of the tow vehicle.