The way i see it is on the same theme as murdoch, if you are making alteration or additions to the circuit then your required to ensure the circuit is safe, this does not imply that you have to run an earth or rewire it, if you can confirm all fitting and switches are double insulated and dont require earthing then you can do the additions to the circuit. With exception of as mentioned before the bathroom as it has its own 'special' location reg's.
As part of the job the appropriate warning lables need to be fitted by or on the consumer unit addressing the lack of earth in the lighting. This way you havent left the lighting unsafe, the next engineer will have warning and the owner will have been told of the situe, having been such a nice sparkie when they muster some spare cash you may get the job yourself.