Hi,
Just recently returned from Kenya and though the above answer is materially correct, you're advised now by the foreign office to have a 'Yellow Fever' inoculation and certificate prior to entry. This is a recent procedural departure, as previously this was only required if entering the country from an infected region.
Yellow Fever jabs cost around forty pounds (including the certification) and I would recommend having it at least a couple of weeks before travel as side effects are possible (It's a live vaccine).
However, no one asked for proof at immigration, so you could always take a chance.
Additionally, Malorone is not now generally prescribed for travel to Kenya due to malaria tolerance. The common substitute offered instead is Doxycycline (Taken every day 1 week before and during travel, and for four weeks upon return), there is a variety which you take once weekly, however there are some issues regarding the extreme hallucinogenic side-effects it can present.
p.s. Don't worry, Kenya is great : )