Fiat currencies like USD, GBP can be printed in Buzz Lightyear's words 'to infinity & beyond' as long as credibility is strong enough to sustain it. USD as the globe's reserve currency, definitely, GBP almost certainly where they are both very, very strong, stable, safe haven in a storm currencies.
Um.....
Since last Monday 9th March Sterling (GBP/USD) has plunged from a high of $1.32 to a low today of $1.20.
Compare that to the Japanese Yen (USD/JPN) which
is a safe haven currency and has gone from Y104.26 on Monday 9th March to a high of Y108.50 on Friday 13th. It fell back a bit yesterday, but has climbed again today.
If I was a conspiracy theorist, I might suggest that some big players are shorting Sterling on purpose to make life even harder for the country in these difficult times, to try and force Boris's hand so he will have to plead for an extension to the transition period. It wouldn't be the first time and a certain Hungarian/American billionaire springs to mind.
But Sterling a safe haven reserve currency? Hardly.