1 addition:

Most countries in the EU require you to mention the VAT amount in their local currency on the invoice. So if you sell something for 100 BTC and the VAT rate is let's say 20%, the total becomes 100BTC + 20BTC = 120BTC and then you'll need to calculate how much 20BTC is in your local currency, GDB in your case) and mention that on the invoice.

example:
------------------------

Product	Price
WidgetA	40 BTC
WidgetB	60 BTC

Subtotal:	100 BTC
VAT %:	20%
VAT:	20 BTC
VAT (GDP):	1428.8227
Total:	120 BTC

------------------------