Hi Lukas, welcome to the forum. In my experience it is tricky to find loose testing in the south of England. Best bet is probably to contact the few rally cross circuits directly and see if you can hire one either as individual or group to share the cost. If you can, you could always advertise on Bristish Rally forum for others to join you.
Otherwise it may be worth trying local car clubs to see what they offer, some have off road auto solos or similar which are cheap, fun and help practice in the car. Here I am at one in my TVR a few years ago, some are faster than others, but any time in the car is good and certainly a good shake down for the car too...
https://m.youtube.com/watch?v=kyeChgOvhrs