Steve Thackery coughed up some electrons that declared:

Hum interesting but I think based on a false premise that the grid
has the *exactly* the same frequency *everywhere*. The frequency will
be close but not exact, the many sets that supply power to the grid
are not connected by a hard physical link but by a relatively elastic
one of the long reactive grid distribution lines.

Not false at all! Every generator connected to the grid is phase-locked
to the grid and is thus bound to run at the same, grid, frequency.

Though I heard from a mate at NG that oscillations are possible (though not
wanted obviously). They had at the old London control centre an instrument
nicknamed the Scottish Wobble Meter. It measured phase differences between
somewhere in Scotland and presumably somewhere the south end of the grid.

I know this because he related one day having to fix it - or rather the
photocopier that was interfering with it causing it to slowly oscillate
giving the control room men an impending heart attack.

On an aside - if you google for National Grid Blackstart you get to some
very interesting documents that show why no-one wants the whole lot to pop.