when a pound is worth €1.45 again. 
Depending on who you listen to, some would say that will never, ever happen again..
That remains to be seen, but if post BREXIT Britain does well and the economy continues to grow outside of the EU and the single market, then the investors will be back and sterling will be in demand again. Add into the mix the fragile state of the Eurozones banks, which this time round includes Germany's Deutchebank and anything could happen if a full blown Eurozone banking crisis kicks off again.

I think that $2/£1 is very unlikely again in the foreseeable future, which is not a bad thing.

It's gone out of fashion these days but governments used to devalue their currencies deliberately to kick start their economies.
What I find interesting is that the currency traders who have been selling sterling are anonymous and unaccountable. Which banks computer caused the so called 'Flash Crash' last week, when Hollande was rattling his sabre's?
