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 style of address is as follows:
|Beginning of letter||Your Excellency (formal) or Dear Ambassador (social)|
|End of letter||I have the honour to be, with the highest consideration, Your Excellency's obedient servant (formal) or Yours sincerely (social)|
|Envelope||His Excellency The Ambassador of X or His Excellency Mr John Jones|
|Verbal communication (social)||Your Excellency should be mentioned at least once in conversation, and thereafter Sir or Ma'am or by name (formal) or Ambassador (or by name) (social)|
*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.