Embassy legalisation for use in Kenya
A reader in Nairobi is not judging whether your degree is good or your marriage was happy. That desk is checking that the paper is genuinely British and that an official acting for Kenya has said so. This is why people legalize documents for Kenya rather than simply posting a copy: the closing stamp is the part a Kenyan office recognises, and nothing earlier in the run speaks to it directly.
So embassy attestation for Kenya is the last event on the file, not the first. Each desk signs off the desk below it, which is why the order is fixed and why two stages cannot run side by side. A paper that arrives at the consular counter without the British certificate gives that counter nothing to endorse.
Where a Cardiff paper starts
What you are holding decides the opening move. Cardiff Register Office at City Hall, Cathays Park, Cardiff CF10 3ND keeps the city's civil records and can print a fresh certified copy of a birth, marriage or death entry. Send that copy on its travels and leave the certificate in your drawer, because a lost civil record can be put right only by ordering another.
A degree, a police record, a board minute or a signed statement takes a different opening. Those papers carry a private signature rather than a registrar's, so notarisation by a notary or solicitor gives the next desk a signature it already knows. That is the practical split between the two starts, and it is worth settling before anything goes in the post.
Cost & timing for this route
The bill comes in two parts. The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX charges around £45 for its certificate, and the consular fee on top is set by the mission rather than by any British price list. Budget for both, and treat couriers as a line of their own.
Timing splits the same way. The British leg runs 7–10 business days, or 2–3 business days if you buy the express route, and that is the part of the calendar you control. The consular stage keeps its own diary. Anyone planning document legalization for Kenya around a start date should count backwards from the day the Nairobi office needs the file, not forward from today.
What makes the Kenya corridor easier
One thing works in your favour here. English is in daily official use in Kenya, so the wording on a Cardiff paper is already readable at the far end, and the argument is about stamps rather than about words. A certificate issued in Wales often carries Welsh and English on the same sheet, and the English text travels with it.