I've used 4' x 2' x 2' cages with Ball Pythons. As long as you have proper hides you shouldn't have any issues. Here is how I set the cage up:
(The bottom cage)
I don't have any first hand experience with live moss, and with you being in the UK I'm not 100% sure on what will be available to you either.
Here is a link to a US reptile shop, that specializes in this type of stuff. They even have an employee who posts here on this site who might be able to help you out if nobody else does. (I'll send them a message about this thread)
http://lllreptile.com/store/catalog/...errarium-moss/
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