Well, after all these months I have finally sorted out my space issue using a Wendy house. Here are some pics and details.
First I bought a more appropriate Wendy house, with windows, and rebuilt the roof trusses to accomodate a ceiling
We then added the ceiling and insulated it with Rhinotherm (a more user-friendy version of aerolite)
We also insulated the walls with Rhinotherm and ceiling board
A bit of polyfiller, quarter round, paint and carpet tiles to finish off
Some new cages
And the new tenants could move in!
My ball pythons
My Brazilian Rainbow Boa
My Jungle Carpet Pythons
My Macklotts
...and my Burm
The temperature stays nice and constant - even more so than the house and the ambient temp in the cages is about 26 degrees with humidty of 75-80%. So it seems to be working pretty well. The room is also snake proof, all gaps and cracks have been filled will polyfiller and PU Foam.
I must say thanks to Reptilians for the advice and assistance!