I can't find a snappy quote for voltage but in the UK it has to be 230 with a tolerance levels of -6% to +10%. (so 216 to 253 volts)

It is generally in the interests of power companies to keep it up because they sell more power. I guess that if, as Selby implies, they have cocked up their deals and it is costing more than they are charging then they might prefer to let it fall.

Though the market works second by second, so I guess it's not that simple.