In the UK, many domestic electricity installations use THe PME (Protective Multiple Earthing) or TNCS (Terre (Earth) Neutral Combined System).
Only two cables come into the property, the generating utility neutral and the single phase live.
In older systems, the lead sheath of the supply cable provided the earth or the the earth was provided by an earthing rod.
With PME, the neutral line is connected to ground/”soil”/earth at locations like sub-stations.
So, the neutral is at the same potential as earth.
At the premises, an earth cable is connected to the neutral at the company distribution equipment, so now three cables enter the premises. The earth cable is used to all appliances as just that.
The neutral and live connect to an RCD (Earth Leakage) trip device. If, in the premises, a fault develops such that the current flowing in the neutral and live lines flows to earth, an imbalance is generated in the currents flowing through the RCD and the RCD trips at its rated trip current, say 100-mA or 30-mA.
If the property is correctly wired, the earth and neutral are at the same potential and are essentially from the same cable, the difference being the earth does not go through the RCD.
However, you never use the earth as the neutral for all the reasons others have pointed out. The only way it would work is if you have no or a faulty RCD, in which case you run the risk off all earthed metallic appliances becoming live.
With regard to the lack of neutrals at light switches, it depends entirely how the lighting circuit is run.
A UK lighting circuit is run as a radial (star) circuit as opposed to a power ring circuit.
If the ceiling rose is used as a junction, then the light switch is wired as a “drop” with a permanent live from the radial and a switched live back to the rose where the neutral for the lamp is.
If the light switch is used as the junction box, sometimes seen in two way lighting, then you will have a neutral available.
When a dimmer or lighting programmer is used to replace a conventional drop “plate” switch, there is no neutral to power the electronics. Instead, the neutral comes from the neutral at the lamp and the current to supply the dimmer etc. relies on a small (milliamp) current passing through the lamp even in the “off” position. If you remove the lamp, the dimmer etc. does not work. This is why many dimmers etc. specify a minimum lighting load.
This was fine when all lamps in domestic properties were filament and purely resistive. The complication arises when electronic loads like CFLs or LEDs are used. These are not resistive loads and may not pass any or very little current to power the dimmer. I had a programmer that worked fine with three CFLs. When I changed the CFLs to LEDs, the LEDs would not turn off or flickered. The only way it would work was to have one CFL and two LEDs. This was not acceptable. The solution was some electronic wizardry where I substituted a resistor (18K) for the lamp. Luckily I had a neutral to work with. An optoisolator (HCPL3760) detected when the dimmer etc. was on or off. A logic output from the HCPL3760 powered a relay. Thus any load from zero to megawatts could be controlled.
If I was rewiring my house, I would provide a neutral at every “node”, rose or plate switch by the simple use of 3+E cable instead of 2+E, or better still, run everything at 12 or 24 volts DC