A barn conversion to wedding venue
Can emergency lighting be wired in PVC twin and earth, and does every light have to a test switch next to it or will one switch by the mains position be sufficient

I would suggest you ask the local council what they specify.

Most EM lights are fed from the nearest normal lighting circuit in twin and earth. Its pointless wiring non-maintained, standalone EM lights in fire resistant cable as this is self defeating, in that if there is a fire the cable will not melt, so the supply to the light is still maintained, so the light will not illuminate.

Since the idea of a test switch is to remove the supply from the EM light it would be advisable to have a test switch near each EM light, other wise you could have a large lighting load going through one EM test switch

You could have all the EM lights on the same circuit all connected via twin and earth back to the consumers unit and have one test switch, but this means the CU would have to loose power for all the lights to illuminate. (It would also cost more due to the added expense of more cable) where as if each EM light is connected to the "local light" if this fails then the EM light will illuminate.

As green-foam has said ask the local council or enforcing authority what they specify, because you may need some form of licence.

Because it is likely to be classed as an entertainment venue and the normal lighting is likely to be switched off when the premises is occupied ( last dance and such) you will need a maintained system which remains switched on all the time the premises is occupied.

To test it you only need to switch off the mains normal lighting at the switchboard to check if the EL is working correctly and the wiring will depend on the design of the system.

Check out http://www.firesafe.org.uk/emergency-lighting/