Why is the Switch disconnector 2 pole device's operational voltage rating at 415V?

Yes, no problem

Yes, the single phase L-N voltage in the Uk is 230V

However, why assume this device can only be connected L/N. You might want to connect it to two phases of a 400V system 

Or between one phase and neutral of a 400/690 volt installation.

The manufacturer will advertise the maximum rating it can be used for so as not to limit its sales. I would be happy using this on a 230V single phase system or between phases in a three phase 400V or 415V system (but not between phases on a 440V system!)

However it is also possible that for a single phase system you only need a single pole SD. I would be more likely to be looking for a two pole SD to fit between phases than between phase and neutral.

Or between one phase and neutral of a 400/690 volt installation.

Seconded, it is made by Germans, their factory planners are more open to this sort of thing, and boxes marked with a big spark sign und “ Achtung Lebensgefahr” ( - translates as "Attention -  Danger to Life" ) do not seem to spook their health and safety folk in the way it would in the UK.

The label says keep your fingers out, so you do. Non compliance will not be forgiven, well not by the electrons anyway.

I have even seen 690/1k2 on a big site, but that did have transformers at both ends and some funny 24V and earth interlock. Funny to see so many kw on a thin cable.

Also it's probably of identical construction to the 3- and 4-pole versions in the same … which would have to 400V (nominal) rated.

Alternatively it might be officially 230V (nominal) rated, but subject to some careless some copy & paste when typesetting…

   - Andy.