There is a British Honorary Consul on Sal, his name is Albertino (Tino) Mosso.
Tino is based in Santa Maria on the island of Sal, Cape Verde.
A brief intro to Tino Mosso can be found here : Link: Tino Mosso - British Honorary Consul
Tino can be contacted by British Nationals in the case of serious danger, death, imprisonment and other similar problems.
Any other issues will or can be dealt through the Portuguese Embassy, who have an agreement with the UK.
You should also look at updates provided on the Government website: LINK: FOREIGN-TRAVEL-ADVICE-CAPE-VERDE