[Red to black]

If you have a relatively low Earth loop impedance (sub ohm) you will probably be already on a TNC-S system, practically all new public supply systems are now this as it is cheaper for the DNO to roll out.
No new TNS systems are being supplied, even existing ones are to be treated as PME (worst case) due to network repairs in the road so to speak (old lead sheaths breaking down and being repaired by being bridged with the N underground) which results in a sort of hybrid system as there may not be a CNE (common Neutral Earth link) in the actual service head in the property.
Old overhead TT systems are also being converted to PME, look for the Earthing conductor/rod applied to every third pole (or thereabouts) on the overhead systems.

I have had such information from the likes of UK Power Networks and others to this effect as we keep in touch with these bodies on an on going basis.

It may well be that if you happen to be "rural" that they may keep it as TT and are just using the materials they are rolling out elsewhere, obviously I cannot say with absolute certainty, but that has been the jist from what I have gathered so far on dealing with various DNOs over the last few years.