"The Tory party conference has given the currency markets plenty to worry about: "
Exactly that, and currencies are only worth what the market confidence in them is. It didn't matter that the new Roman coins had more silver in them around 440AD than previous issues. People took one look at them, then at the old ones and within 18 months roman currency was valueless as no-one would accept it.
Flash crashes happen because people game algorithms to try and gain an advantage (the algorithms are in turn setup by people who might be good at statistical analysis, but they don't know all the inputs, and the people using the algorithms know just enough about them to clock-n'drool), OR because something is genuinely overvalued and once the bubble starts to burst the automated activity rapidly drives the trade values back down to "actual" levels.
After WW2, "British made" became a codeword for "shonky designs-by-committee using the cheapest possible supplies regardless of quality and put together by badly treated workforces using century old production tools" (literally that old in many cases) - the epitome of this being British Leyland. That's why people of the "empire" stopped 'buying british' unless forced to by their governments. Even though the quality has changed, memories are very long - as GM found when it tried to resurrect the Vauxhall brand in Australia/New Zealand.
The only thing that's been keeping the value of the pound high over the last 4 decades is the financial services industry. When that industry lost confidence in its home base - it has, and all the orgs are already moving their bases out of the UK - the pound lost a vital support base. Its remaining value comes from exports by its manufacturers - who mostly happen to be the car industry and also spooked into moving out. Commissar May could stand up tomorrow and say that Brexit is cancelled but it's already too late.
But never mind. At least we got that £350million to spend on the NHS, didn't we?