I intend to take feed from kitchen ring main through a DP (...)

I forgot. If the shed has any structural metalwork or metal services, such
as plumbing of any kind, they will need to be main bonded back to the main
earthing terminal near the consumer unit using an unbroken cable of very
large cross sectional area. As you say the consumer unit is inaccessible,
this would preclude your technique and require a TT installation.

Christian.