OCPD

If you mean RCCB, and not RCBO, then really there is nothing to discriminate with. The RCCD== RCD  only picks up earth faults  - as without it you and the DNO would both need one heck of an electrode to blow a 100A fuse promptly - so that and the electrode need  testing, and the 100A fuse only picks up L-N faults.
Arguable its a pain in the pinny and poor practice if the only fuse is the DNOs, so an RCBO would be much better, but I'm seeing no actual danger here. C3 at most. So long as customer is happy to call out DNO if there is ever an L-N accident with whatever is plugged in - which they may not be.
Mike.

100ma RCCB at the unit
:
system is TT

What's the wiring system between the cut-out and the socket like? Is there a c.p.c. alongside live conductors before the RCCB, either before or within the socket? I'm just wondering about the possibility of earth faults before the RCCB...

everything within 3 meters of main head

And does the wiring meet all the DNO's requirements for their fuse providing protection (typically there's a stack of requirements including type of cabling and disposition as well as length) - or failing that BS 7671's section 433/434 requirements for omitting protection (if, lacking the DNO's agreement, the DNO's device technically can't be relied upon) - roughly ≤3m, installed to reduce the of faults to a minimum and risk of fire or danger to persons. Often the DNO's requirements boil down to something very similar.

does there need to be means of isolation and OCPD between cammando socket and main head?

Depends on operating requirements... would the customer find it acceptable to have to call out (and pay for) the DNO every time any kind of work is needed on the installation (probably including period inspection - insulation tests etc normally require the circuit to be dead).

   - Andy.