We seem to be going round in circles, because we don't know your motivation in asking the question and hence what parameters you are aiming to specify. Do you have equipment that you want to operate in the UK that expects such a supply? Or do you want to apply BS7671 to an installation outside the UK with this supply configuration? Or is it purely an academic question?
The UK LV electrical supply is standardised as 230/400 with one leg of single phase supplies at or near ground potential, for general applications. For specific purposes, you can use whatever voltage and configuration is necessary, but not for the general wiring of buildings. The specification of the supply delivered to the consumer's terminals falls outside the scope of BS7671, governed instead by the ESQCR.
Portable appliances are also outside the scope of BS7671 and must achieve pan-EU compatibility in order to be CE-marked and hence permissible to sell. As per posts 19 & 29, a portable appliance has double-pole isolation inherent when unplugged, so does not require special provisions for compatibility with supplies of unknown polarity.
The UK LV electrical supply is standardised as 230/400 with one leg of single phase supplies at or near ground potential, for general applications. For specific purposes, you can use whatever voltage and configuration is necessary, but not for the general wiring of buildings. The specification of the supply delivered to the consumer's terminals falls outside the scope of BS7671, governed instead by the ESQCR.
Portable appliances are also outside the scope of BS7671 and must achieve pan-EU compatibility in order to be CE-marked and hence permissible to sell. As per posts 19 & 29, a portable appliance has double-pole isolation inherent when unplugged, so does not require special provisions for compatibility with supplies of unknown polarity.