Thank you for posting here in the Community. I can help walk you through the steps on how to change the currency to your customer.
Please note that once you turn on the multicurrency, you can't undo. Also, you're unable to change the currency for an existing customer. You'll need to create a new one with a different profile and be sure to select the foreign currency during creation.
Let me guide you how:
Go to the Gear icon located on the upper right side.
Click the Advanced tab.
Hit the Edit button perpendicular to Currency.
Choose your Home currency in the drop-down arrow.
Put a check marked on the Multi-currency, then tick the box in the I understand I cant undoMulti-currency.
Once done, you'll need to Make Inactive the customer's profile so that you can update the customer's currency. Even though a customer is made inactive, they will remain on reports of transactions related to them.
Go to the Sales tab, then select Customers.
Double click the customer's name and click the Edit button.
Press Make Inactive, then click Done.
4. Go back to the Customers section and hit New Customer.
5. Then re-create the customer's profile.
6. Tap the Payment and billing icon.
7. Click the In this customer pays me with drop-down, then pick GBP.