British Ambassador Accredited to a Foreign Country
A British Ambassador accredited to a foreign country is known as 'His/Her Excellency' within the country to which he/she has been accredited (and often by courtesy when travelling outside it on duty), but not in the United Kingdom.
Similarly an Ambassador who is Head of a United Kingdom Mission abroad (eg to the United Nations) is styled 'His/Her Excellency'.
A female Ambassador is called Ambassador, and not Ambassadress. Her husband is not accorded any style as such.
How to Address a British Ambassador
The recommended social style of address is as follows:
|Beginning of letter||Dear Ambassador|
|End of letter||Believe me, My dear Ambassador
|Envelope (if a peer)||His Excellency the Viscount Flintshire, KCMG*|
|Envelope (if a Privy Counsellor)||His Excellency the Rt Hon John Jones, CMG*|
|Envelope (if a Knight)||His Excellency Sir William Smith, CMG*|
|Envelope (untitled gentleman)||His Excellency Mr John Brown, CMG*|
|Verbal communication (social)||Ambassador (or by name)|
*If desired 'HM Ambassador' may be added, followed by British Embassy (except where the residence is not within the Embassy).
There is just one British Legation overseas, headed by Her Majesty's Minister accredited to the Holy See, who is styled His Excellency and addressed in other respects like an Ambassador with the substitution of Minister.