You can invoice for the goods and services that you supply in any currency. However, if there is any UK VAT due on the transaction, then your invoices must also show the following in sterling:
- the total net value of goods and services at each VAT rate
- the amount of VAT, if any, at each rate
You do not need to show sterling figures for each line on the invoice.
The value in sterling that you show on a VAT invoice must be the value at the time of supply. However, if you issue invoices before you actually make a supply then the date of supply may be in the future. One example is leasing or rental payments. Clearly, you won't be able to convert future foreign currency amounts accurately into sterling, since you don't know what the exchange rate will be.
Advance invoices must therefore be in sterling for VAT purposes, and those sterling amounts will be what you'll have to enter into your VAT records. You can also show your invoice amounts in a foreign currency, but you'll have to make it clear that for VAT purposes, the value of the supply is the sterling figure, and is not a conversion of the foreign currency amount.
Read more about how you convert foreign currency transactions into sterling.