Thanks for your replies. No, we couldn't run the cable round the house as it's not detached, It's a townhouse in between two others
Not too sure on the Earthing system, sorry.
No metal fittings or copper pipes in the cabin.
6mm only because I've done a bit of research prior and from what I can see majority say use 6mm cable or above for future proofing (I've probably misunderstood)
This is what I was planning to fit in the cabin:
British General 5-Module 3-Way Populated Garage Consumer Unit - https://www.screwfix.com/p/british-general-5-module-3-way-populated-garage-consumer-unit/1926g#product_additional_details_container
again, thanks for your help.
PS- I think I entered my DOB wrong. I'm 31, not 21

Wish I was!