Not wishing to dis some of your advice, but struggling a tad with some.
Metal services must be bonded. Plastic services do not need bonding, and wouldn't waste my time checking them. The metal pipe work that runs from the plastic services may need bonding (528.3).
Changing from TT to TN-C-S may have a cost implication to the client which may not be necessary (although I would), and I wonder why installing 100mA S Type in this situation, would be a consideration. And OP doesn't have to test the whole installation, if he is doing an alteration or addition (minor works).
I'm always learning, so I willing to stand corrected.
might have read slightly wrong with plastic pipe & not mentioning the metal part after.
changing from TT to PME or TN-C-S has always been free for me & have done quite a few
the 100ma trip S type is normally used to protect cabling in the infrastructure more important in what was 16th edition as lights ect did not require RCD protection now most circuits are 30ma protected. but I would always do a TT system with 100ma trip & the reason for s is the time delay so the 30ma will trip first if you only use 100ma without S type they can go at the same time as the 30ma & take out whole board.
me now with metal clad board cables in there unprotected I would install the 100ma trip on a TT.
testing meaning the one circuit or more if pumped shower (running of sockets) & lights running of lighting, i would want to test the whole circuit before i put an RCD at the mains hence the mention of RCD spur above the door & protecting just the work i would be installing so i don't have to test the whole circuit.