I'm curious, if you do have an earth fault on the lighting circuit
(lets say the earth goes back to the consumer unit earth bar). If
nothing trips how will anyone know that the fault has occurred. Surely
it can't run like that indefinitely?

In the real world, an earth fault will be a dead short, not via a light
bulb. Therefore, there will be a dead short between the lighting circuit
live and the earth. When designing the installation, the earth loop
impedence is calculated, which gives the expected earth fault current. This
must be more than 5 times the MCB rating for the circuit. An MCB will
disconnect instantaneously at this multiple.

All circuits should be designed like this, even socket circuits. The RCD
then provides supplementary earth leakage protection to the socket circuits,
as it is more likely to have a non-short circuit earth fault, such as a
child sticking a screwdriver into the live whilst touching a radiator.
However, in the event of a dead earth short, both the MCB AND the RCD should
have a chance of cutting the power.

I suppose plastic switches would be required until I get it
properly earthed.

Yes, you may have to wait. Alternatively, I heard a rumour that some ranges
of metal light switches are available double insulated and do not require
earthing.

Christian.