If you are using currency resolver module in d8 you might need to change resolved currency quickly go to
and in getCurrency method, put
$this->currency[$request] = "EUR";
for example, to get EUR currency in your checkout.
To quickly change/simulate IP of the user go to same namespace, check CurrencyHelper class and fing getUserCountry method, in there, before return you can enforce your own abbreviation like
$country = "GB"; return $country;
and you will enforce custom country IP.