Simply select the UK as the country and enter the BFPO address in the postcode box. As advised by the Royal Mail, everyone's security is important so please do not write the country of destination on the address.
An example of how a BFPO address should be entered can be seen as follows:
Name: 23456789 Pte Green
Street: A Company
Town: 1st Battalion
City: The Postal Rifles
Postcode: BFPO 123
Country: United Kingdom
