They are among nine new posts in Commonwealth countries as part of an expansion of the UK's overseas network announced by the British Foreign Secretary Boris Johnson.
Speaking at the Commonwealth Heads of Government Meeting in London, Mr Johnson linked the announcement with the UK's planned exit from the European Union.
He said the new diplomatic posts were in regions which provided "huge potential and opportunity post-Brexit for British businesses".
Britain already has a high commission in Fiji.
A statement from the UK government said the expanded network "puts Britain in peak position to enhance military cooperation, boost trade and provide services to the rapidly expanding middle classes of the developing world".
This comes as Tonga's Prime Minister Akilisi Pohiva yesterday described the UK's re-engagement with the Pacific as "music to Tongan ears".
However, Mr Pohiva's offer to return a property to the British Government in anticipation that it would open a new post in Tonga has caused a stir as a Tongan royal office is currently located on the property.
Meanwhile, Mr Johnson said the diplomatic expansion was part of the Commonwealth's collaboration in boosting prosperity, tackling security issues and clearing up the environment.
This follows the establishment of a new alliance under the leadership of the UK and Vanuatu aimed at fighting plastic pollution of the Pacific and other oceans.
The British government has committed $US88 million to the Commonwealth Clean Oceans Alliance.