Current price for a basic 20ft steel shipping container is in the order of £1500 plus VAT and shipping. Box will weigh in the order of a ton. So access to remote areas needs thinking about.
Try contacting Plant hire companies to see if they're selling of kit.
We have bought used kit from a Company in Gloucestershire called Severnside relocatables.
http://www.relocatableportablebuildings.co.uk/Our_Range/Modular_Building_Relocation/We got an unrefurbished office with toilet/washroom for around £2K a year or so back.
Use a delivery firm with a Hyab on the truck for off loading.
Cheers.
Andy