That is not the declared voltage, it is the nominal voltage, something which some jobsworth in an office has written down and is impossible to implement. Nothing has changed because it cannot be changed, the whole country has substations with their output fixed at 250/433, there's nothing any jobsworth penpisher can do about that!
It doesn't change the basic physics though, If the stated power rating is at 240V you cannot just replace 240 with 230 in the calculation. The power dissipated will be less at 230V so the current which flows will be less.