Hi Kamila 
I purchased many things online in a foreign currency and I always use Monzo for it. Books from Bookrage.org in PLN (Polish company), I paid for my email (Protonmail) in USD, my Usenet account is billed in EUR, I even have one subscription in Australian Dollars too. Never had any problems! 
Make sure that you are always billed in purchase currency (so USD, PLN etc, and not billed in GBP), as some payment gateways try to apply their exchange rate. This problem sometimes happens with Paypal payments. This is the online equivalent of ATM’s DCC and here’s shameless plug explaining this. I found that it’s not common, much less of a headache than ATM withdrawals. 