A friend of mine bought a used van. It is an older model but has tons of room. He is thinking of gutting it to make it into an off-the-grid livable space for when he goes camping and on road trips.
I don't know if there is a cheaper or more cost-effective way to do something like this as I have only ever seen the process being quite expensive.
I don't know what the costs of something like this would be here in the UK but this can give you an idea of the process:
Here is another guy who did something similar in Australia:
I don't think this is a "cheap" process but it is certainly cheaper than buying a home or renting.