Hi, what you need to do to comply with section 7 of the 17 th edition regs is fit a 32amp breaker in place of that spare 20amp one in the house, run a length of 6mm twin and earth to an adaptable box and convert to steel wire armoured cable to run to the caravan. The swa cable need only be 2 core 6mm as you need to segregate the earth at the caravan end and fit an earth rod and connect that to the caravan disboard with some 10mm green and yellow 6491x cable, then test your earth loop impedance and make sure it's under 200 ohms.
Also make sure you connect the steel armour to the house earth only
Cheers Jason