Which currency you are billed in
Horizon bills in euros or pounds sterling. Both are enabled.
How your account starts
A new account starts in the currency its country suggests. Accounts in the United Kingdom, Guernsey, Jersey and the Isle of Man start in pounds. Everything else starts in euros, including an unknown or empty country.
That is only a starting point, not a verdict.
Changing it
Go to “Settings”, then “Language & region”, and use “Currency”. On the “Billing” page, the wallet card for the other currency says which one the account uses, and its “Go to Settings” button opens “Language & region” directly.
Once you have chosen, the choice sticks. Later changes to your billing address will not move your currency back, because an address change is not a decision about how you want to be billed.
What pins it in place
Two things make the currency stop being freely changeable.
A live subscription binds it. The price you are subscribed to is denominated in one currency, so the account has to keep displaying and charging in that one. When you change plan, the new plan’s price is resolved in your subscription’s currency, not in your display currency, and the change dialog says so when the two differ.
An existing record with your payment provider also binds it. Once your account is mapped to a currency there, it cannot be moved. This is worth knowing because a first payment attempt that failed can still leave that mapping behind.
When a quote no longer matches
If the currency you were quoted is not the one the account would actually be charged in, the purchase is refused before your card is touched, and the catalogue is re-quoted in the right currency.
You are shown a notice explaining that prices have changed currency, with the new figure, and you confirm again. Nothing is charged by the refused attempt.
This is a safeguard, not a fault. It exists so that you are never charged in a currency other than the one you agreed to.
Amounts are never converted
Horizon does not convert amounts between currencies for display. A figure shown in one currency is a figure that was set in that currency.