TNS, TNCS and TT are the three supply types most commonly found in the UK. There are two others.
This type of supply has no direct connection to Earth. The energy source (transformer or generator) has no connection to earth at all (Isolated). Very uncommon, one example would be a small portable generator with no earth rod.
Do not confuse this supply type with computers or Information Technology. Some buildings may have special supplies for computers, but those are completely separate topic.
This is where the neutral and earth conductors are combined throughout the installation - i.e. no separate earth anywhere. This is not permitted in the UK, but is used in some other countries.
These will be decades old and probably undersized. Rubber insulation dries out over time, and can look perfectly fine. Touch it and it crumbles to dust. Rubber or cloth insulated wires should be replaced.
Nothing wrong with bare earth wires, but if found in domestic situations they are probably very old and far too small.
Water pipes could be used as the main earth - provided it was done before 1966. Even if the pipe provides a good earth today, the pipes in the street could be replaced with plastic ones tomorrow.
Gas pipes were never permitted to be used as the main earth.
In both cases, an earth should either be provided via the DNO or an earth rod.
Main equipotential bonds do connect to the water and gas pipes, but this is NOT to provide an earth, but to ensure the metal pipes within the installation are at the same potential.
This should never happen, but of course, it does. Perhaps it is a TT supply and the earth rod was damaged or removed. Maybe the earth clamp on a TNS fell off and was lost. Or on a brand new TNCS supply, the earth was never connected.
Regardless of the supply type, an earth must always be provided, either by the DNO or the property owner. Without one, there is little or no protection in the event of a fault.
Even if the main earth connection looks fine, it will still need to be tested. Underground cable joints can corrode away and fail, and the earth terminal on a cutout might not be connected to anything inside.