Simples .....
1)
Law (e.g. Building Regs) don't do what it says and you can be prosecuted!
2)
Guidance (e.g. BS7671, OSG, GN's, Electricians Guide to ....) don't do what it says and you can't be prosecuted.
However, by following 2), should you be prosecuted in Law under say the EAWR's you can offer the likes of conforming to BS7671 in your defence! (Not too sure if the guide to the building regs has the same sway as BS7671 but I would doubt it)
For the 300mm case, if actually stated as that distance in the building regs (I assume it means horizontally from the sinks edge), if its placed horizontally 250mm from the sink edge but 2m up the wall, in law you could be prosecuted -- i.e. it either is or isn't within 300mm horizontally of the sinks edge. The fact thats its 2m up the wall would be mitigation in your defence but your still within the 300mm that the law requires.
If it was just guidance, you could argue the 2m up the wall negates the 300mm guidance -- it could be directly above the sink and well out of harms way.
Hence my original query as to whether the 300mm is actually stipulated in the building regs as a distance or just someones interpretation/guidance on what they think something in the building regs about accessories in a kitchen means!
The law may be an --- but if you don't follow it beware ..... Q.E.D.?