I think back to some of the systems I have seen, which includes a suspended floor in Liverpool with TT system and the earth rods were under the floor, there were plans for an assess hatch, but not a clue if ever built.
But it seems in order to ensure you don't have a TT and a TN in two next door premises so you can have a voltage difference between the two, the DNO has to say what the earthing system at the head will be, OK I do know of some exceptions where the owner was a radio ham and the whole house was TT but all neighbours were TN-C-S but this was an odd one out, so all it should take is a phone call the the DNO and they should tell you.
Yes it does go wrong, my son rewired a house in Queensferry North Wales, and he phoned the DNO to find earth type, which he was told would be TN-C-S so he left the tails for a TN-C-S but he was in middle of a divorce, and moved out of his house and went to live in a narrow boat, so of no fixed abode, the result was he was never told when the DNO connected, and out of interest some months latter visited to see how it went, only to find no earth had ever been connected, the DNO had changed their mind and given a TT supply. Earth rod was then fitted.
My parents were the same, I know there was an earth, as when a kid I had caused a short to earth and blown the fuse, but some 40 years latter no sign an earth had ever been connected. Only earth was for the old party line phones fitted by GPO. A phone call and DNO did FOC fit an earth making it a TN-C-S supply.
But what this all means is you can't guess, in the main below 0.35Ω it is TN-C-S, below 0.8Ω it is TN-S and above 0.8Ω it is TT with domestic premises, but it does not always follow, and on commercial premises it can be much lower, if anywhere on the supply transformer to house earth and neutral are combined it is TN-C-S it does not need to be combined at the head, so you may see it is TN-C-S but you can never be sure it is TN-S unless the DNO tells you. So there is really no option you have to phone DNO.
