Insulation Thickness Query

I am looking to build a small camping trailer just for sleeping in when I go away to model plane flying events. What is considered the average insulation thickness to sustain relatively warm conditions inside the trailer un-heated? I see 3/4" used here a lot, I am thinking of 1" ridgid foam insualtion all over. The trailer will be 4'w x 6.5' long and 3' tall. I am based in the UK so camping from 10'c to 20'c on average.