So British currency is Same as Zimbabwe currencies the europe knows that and they Don't want to Exchange British currency also the USA Don't want British currency.
Brexit was all about save european union!
On the contrary to this, the brexit wasn't as you said it. The EU as a group wanted the EU currency to be one and stronger by trying to get Britain to drop pounds and adopt only Euro.
The issues in EU block have been a big one during the reign of many British Prime ministers like
Tony Blair( 1997-2007, labour party),
Gordon Brown (2007-2010, Labour party),
David Cameron (2010-2016, Conservative Party),
Theresa May (2016-2019, Consecutive Party).
and now Boris Johnson.
The brexit issue became serious during the Theresa May administration which was the main reason she resigned.
Therefore, the EU know that the Euro will be stronger if they have one unit currency.