Yes, the British IEE usually considers 250V to be something of a cut-off point above which extra precautions need to be taken. (250V being the highest nominal voltage in the days when different districts could have slightly different supply voltages).
There is nothing to stop a very large house having a 3-phase supply, but it's very rare. If 3-phase is supplied, then all sockets in any one room must be on the same phase, the idea being to eliminate the chance of two appliances within arms reach of each other having ~400V between them.
Even in commercial environments there are requirements for labeling of the highest voltage present between adjacent equipment if that voltage exceeds 250V.
Whereas in Continental Europe most of you can have residential 3-phase service at maybe only 20A per phase, in the U.K. we prefer 1-phase at 100A or so for residential.
Does anyone have any views for or against either approach?
