The SLU won't do anything until you drive the car, it works by 'pumping up' every time the car dips low on the back end. Try the HD 'police spec' rear springs, they are progressive and the full set are about £50 for 4.
Don't just fit spacers if you tow, the standard rear springs are quite soft because of the SLU, a Disco without SLU has heavier springs, these are another option for you.