The treaty route, and the one Cuba keeps
A country can accept a foreign signature in one of two ways. It can join a treaty and take a single slip from every other member, or it can keep the older arrangement and have its own officer look at each file. Cuba keeps the older arrangement, which is why one British mark is never the end of the story.
Where Cuba stands on the Convention
Cuba has not joined the Hague Apostille Convention. A paper going to Havana, Santiago de Cuba or Camagüey therefore requires embassy legalisation on top of whatever mark it already carries. Britain has been in the treaty for decades, which is why the slip exists here at all, but a treaty binds only the countries that signed it.
From Cardiff to a Cuban desk
The British stage is quick and impersonal. One office, one fee per paper, no interview and no counter in Wales. It marks a register copy, a court copy or a paper signed by someone already on its list, and it posts the file back to you exactly as it arrived.
The consular stage that follows is the one that decides your calendar. An apostille from United Kingdom to Cuba reaches a person who reads the actual document, so the file needs to be right before it goes. Sending the wrong version means starting that stage again, and it is the slower half by a wide margin.
Spanish, and the papers that make the crossing
Every public office on the island works in Spanish, so an English paper with two marks on it still needs its words rendered. Do that once both marks are in place, so the translated set describes the finished bundle. A translator normally reproduces the marks alongside the text, which is what a Cuban clerk expects to see.
The traffic out of Cardiff is mostly personal rather than commercial. Marriage and birth records, single-status papers and police records make up the bulk, driven by families and by couples settling on the island. Company files are rarer. Where the paper is a civil record, order a certified copy first and keep the one you already hold.