You can buy free standing aircon units from B&Q etc. The hum will keep you awake at night & other campers as well who will pull out your ehu so they can sleep. Theres no mileage in aircon units in tents. If you are too hot at night save yourself some money & don't drive so far south.
Wet your bedsheet in cold water, wring it out, and you'll be plenty cool enough in bed underneath it as the water evaporates. Works a treat in the Med (I hate the humming of air con), should work in UK.