Berlioz,
I am a qualified engineer I work freelance doing electrical & mechanical designs myself, however I choose also to install and build my own designs.
I am sufficiently qualified to practice as a professional engineer in Europe and many other countries in the world even where a licence is needed to practice.
I chose my own destiny what I do and who my customers are.
I have a group of reliable suppliers and sub-contractors I can utilise as I choose.
I am fully aware of the basis of BS7671 on the IEC standard, as I teach the wiring regulations and the inspection & testing of installations qualifications.
I also understand ohms law, the Kirchoff laws and several other fundamental laws of science.
The UK regulations differ in more than just traditions and devices.
By the way I appreciate that English is not your first language, but I do find your posts difficult to understand, which is made worse by your typographic style.
The NEC which is prevalent on the American continent differs quite a bit from the IEC standards, and both differ from BS7671.
We impart many of our own regulations onto the standard, this has become even more transparent with the last amendment in the re-numbering of our UK national regulations to make them stand out from the IEC harmonised regulations.
If you are as you say a designer then I am disappointed that you undertake such a simplistic calculation of circuit loading without considering any external influences on the installed circuit.
As I understand things there are differing installation methods in the IEC standard, and there are influences of insulation of cables and other such de-rating factors included, thus these must be considered to ensure that you have a safe and compliant design.
Electrical designers in the UK tend to work on larger projects. Small domestic, commercial and industrial works are normally designed, installed, inspected and tested by the installer, as this is taught during their training.