There are a number of ways to address a British ambassador depending on whether the communication is verbal or written. It is also important to know how to address the spouse of an ambassador. British High Commission and Consular diplomats also have specific forms of address.
Begin a letter to an ambassador with "Dear Ambassador" and end it with "Believe me, my dear Ambassador, Yours sincerely" followed by your name and title. Address the envelope to "His or Her Excellency" followed by the ambassador's title and name. For example, a peer is addressed as His Excellency the Viscount Monmouth, KCMG. Whereas, a knight is His Excellency Sir John Bull, CMG, and an ambassador without a title is His Excellency Mr John Bull, CMG. The ambassador is only addressed as His or Her Excellency when overseas. This address does not apply when the ambassador is in the UK.
Address the ambassador as "Ambassador", or by name if the situation indicates this is appropriate, in verbal communication. For example, if the ambassador asks you to call him by name. Please note that a female ambassador is not an ambassadress and you should address her as "Ambassador".
Address the husband of a female ambassador according to his personal title, such as Mr or Sir, as he does not have any special form of address. The wife of an ambassador who is a peer or knight, whether the title is hereditary or a life peerage, has the title she would have if he wasn't an ambassador. For example, Viscountess Monmouth is Lady Monmouth. The wife of Sir John Bull is Lady Bull.
Use the same form of address for Commonwealth High Commissioners as you would for a British ambassador. This post is equal to that of an ambassador in the London diplomatic list. For example, the High Commissioner for Australia is His Excellency Mr David Jones, High Commissioner for Australia. Start letters with "Dear High Commissioner" and end with "Believe me, Yours sincerely". Address invitations to The High Commissioner for Australia and Mrs Jones. In conversation, address him as High Commissioner.
Address letters to a British Consul-General, Consul or Vice Consul as John Bull Esq, CMG, HM Consul-General. A Consul is John Bull Esq, HM Consul, and a British Consul is John Bull Esq, British Consul. Only those posts with Her Majesty's Commission are entitled to use HM before the position. Any post not appointed by the Crown doesn't use the prefix HM.
Check the ambassador's name and its spelling before writing. Call the embassy for help. Check the ambassador's spouse's name with the embassy before issuing an invitation.