"if it had plummeted pre-Brexit it would have been a negative thing so not sure what's changed?"
nothing's changed, i repeat: we are immediately poorer now because we can buy less foreign things. things like fuel and food. or anything, really, but especially things without a British alternative.
but this reckoning had to happen eventually, one way another, because you can't sustain a permanent outflow of good and services without eroding your purchasing power, and this fundamental truth, which has been laid bare by the political uncertainty of brexit, may well suppress the good ship Sterling further yet.
exports are not "more fluid", but they are now "cheaper" in foreign money. and yes the converse happens for imports. foreign things now cost much more than "buying british".
a rich man like you will probably barely notice the price of your cognac and barolo increasing and ed's foreign hookers and drugs will only make a slightly bigger dent in his trust fund than before, but the gross purchasing power of the nation will be significantly reduced at first and then directed inwards in response.
it's not hard to measure in real terms because these things are very closely and carefully monitored, by the inland revenue and the treasury, so these things are precisely known quantities and even the black economy is well studied and quite accurately quantified.
how does that balance in the new world? i though ed conway in the times today put it better than i could:
"Competitiveness is a euphemism for earning less. Having barely recovered from five years of falling real wages, will [the great unwashed] really be consoled by the knowledge that they have helped rebalance the economy?"
hopefully we are taking 3 - 5 years of pain in an attempt to get fit for the next 50.