I have spoken with some expert in this topic and he said that the test key switch is still required even though if the lights are self-testing type as we need to have some point of isolation of the lights for service or maintenance purposes, unless there is a possibility of isolating them directly from the fuse however, the distribution board must be reachable ideally in the same room/area. 

Isolation can be achieved at the distribution board by operation of the MCB/RCBO.

I am led to understand that isolating at the main mcb is not recommended due to inconveniencing other occupants of the building, and that a test switch is the desirable method.

Yes, that's what I mentioned above but apparently the DB with the designated MCB needs to be somewhere close to the circuit or component being serviced. I am not sure how far or how close though. 

It's not a requirement, but that's where the discussions with the client come into play for how they envisage the building operating. The key test switch doesn't normally isolate the circuit, just breaks the permanent live to the emergency fittings. For maintenance you'd still need to isolate at the board, but for phased manual testing, distributed key switches are good. Self test does away with that though.

Tom D said:

I have spoken with some expert in this topic and he said that the test key switch is still required even though if the lights are self-testing type as we need to have some point of isolation of the lights for service or maintenance purposes

But aren't most fishtail switches to the same standard as ordinary plateswitches (e.g. BS EN 60669-1) - so, according to table 537.4, not suitable for isolation?

   - Andy.

Tom D said:

I have spoken with some expert in this topic and he said that the test key switch is still required even though if the lights are self-testing type as we need to have some point of isolation of the lights for service or maintenance purposes

But aren't most fishtail switches to the same standard as ordinary plateswitches (e.g. BS EN 60669-1) - so, according to table 537.4, not suitable for isolation?

   - Andy.

On the ones I have been looking at they have used MK double pole grid switches with the neutral passing through the switches too.

I think isolation is more to do with the clearance and creepage distances - N often doesn't need to be broken for isolation in TN systems.

  - Andy.

Might be worth looking at using a contactor per floor of the building to achieve isolation and then a key switch for manual testing even if it is a Self testing system. The flip side of that is the cost but if the customer is happy to pay