Beware of people that don't know what they don't know
There's some dangerous misapprehensions being propagated on this thread by people that have a GCSE-level theoretical understanding of domestic electricity power supplies. The real world is not so simple and not so predictable ("The difference between theory and practice is that in theory there is no difference...").
It is this level of misunderstanding/incomprehension that has caused to law to be changed to require people working on power supplies (including domestic) to be properly formally qualified. Sad, but probably necessary.
So, if you want to physically interconnect properties, get information from a *properly* qualified electrician.