As TTC has tried to explain, I think it is you that has a misunderstanding.I think there is a misunderstanding there. It is part of building regulations. In a sense these have no force in law but what matters is the outcome. ...
BS7671 (formerly, and still subtitled, the IEE/IET "Wiring Regulations") has nothing to do with the Building Regulations (which are 'the law'). If the Building Regulations had wanted to, they could theoretically have required electrical work to be undertaken in compliance with BS 7671 - but they didn't. Instead, they just produced a one-sentence requirement about electrical work (in the Building Regulations), just saying that it has to be undertaken safely.
In practice, demonstration of compliance with BS7671 will usually be taken as indicating compliance with the Building Regs, so that's the easy route that most people take. However, if one has a good argument as to why the work is 'safe', that satisfies the law (the Building Regs), even if the work does not comply with BS 7671.