there several proposals for ELDO launch site after Woomera launch site was unusable for Europa II rocket
here is the list:
The Seychelles archipelago
Trincomale (Sri Lanka)
Fort Dauphin (Madagascar Republic)
Mogadishu (Republic of Somalia)
Djibouti (French coast of Somaliland)
Port-Etienne (Islamic Republic of Mauritania)
San Marco italy (coast of Kenya)
Darwin (Australia)
The island of Nuku-Hiva Hiva (Marquises, French Polynesia)
The Touamotu archipelago (island of Rairoa, French Polynesia)
The island of Desirade (French West-Indies)
The island of Marie-Galante (French West-Indies)
The island of Trinidad
Cayenne (French Guiana)
Belem (Brazil)
NOTE all French location was also under consideration by CNES for a French Launch Site, in end it became Kourou (French Guiana).
It was fast selection for France
every site to reach true Panama or Suez cancel was rule-out (in case of blockade)
other sites were to remote for infrastructure like The Touamotu & Marquises archipelago
French Guiana was chosen because:
Easy access from France, good launch path over Atlantic
and french Foreign Legion on-site, who build later the site and protect it.
so the French could move there operations away from Algeria, Hammaguir launch site
As ELDO look for Europa-II launch site, the French Kourou space center was ready in use