Ideally you'll want at least 2-3 inches of clearance off the ground for the sake of your PSU and 4-6 for every other side that has any sort of air flow just to make sure the air can travel properly. What I did was got a CPU stand like this one and put a plastic mat under it (like the kind you use for wheeled office chairs) to ease getting it out from under my desk for periodic cleaning/dusting.
I just glide mine across the carpet and my other spare gaming pc is laying under my bed on carpet. I did find me a free mobile
stand though something like this.
Another in wood cabinet to slide off of.
I have mine on one of these raised off the floor.
I've had it for so long I'm surprised I found a link but here it is!
I was thinking of getting something like this
https://www.amazon.com/dp/B001AJS3BS/ref=cm_sw_r_cp_api_HNjPxbW26W4YZ
You think that will do?
There are wheels made just for computer cases. I know you are in the UK but here's a link on amazon usa with some listings https://www.amazon.com/Kantek-CS200B-Mobile-Stand-CPU/dp/B001AJS3BS
I bought a mobile stand that's working out pretty well. That being said it does tend to tip just a bit if you push at the top of the case.