Sharing English is not the same as being trusted
It is easy to assume that a country reading your language will take your paperwork as it comes. Recognition works on a different track. It rests on treaties between states, not on what a clerk can read, and Kenya sits outside the treaty that makes a single slip travel. So the file is easy to read and still needs a second mark.
Kenya's standing on the Convention
Kenya has not joined the Hague Apostille Convention. A paper bound for Nairobi, Mombasa or Kisumu requires embassy legalisation on top of the British mark, and the consular officer expects to see that British mark already in place. The order matters: a file presented the other way round comes back, and the fee spent on it is not returned.
The British half, from Cardiff
One office serves the whole country and deals with everything by post, so there is nothing in Cardiff to attend in person. It marks register copies, court copies and papers signed by people already on its signature list. An apostille from United Kingdom to Kenya begins here and is the cheap, predictable half of the journey.
Because the charge is per paper, the choice of what to send is the real decision. A graduate heading for a teaching post may need a degree and a police record but not a transcript. Ask the Kenyan employer which sheets they want before anything is posted, and the bill shrinks accordingly.
The consular half, and your calendar
The second stage is where the clock really runs. A consular officer reads the file rather than a list, keeps a record of it, and works at a pace set on their own side. Work permits and school terms have fixed dates, so start this stage as soon as the British mark is back rather than in the final fortnight.