I think most of the international schools start reception at age 4 (4 by September). ESF, I think, uses a January cutoff but someone can probably speak better on that. They have kindergarten starting around age three I believe. We opted for a private 'pre-school' for our child until he was age 5 as we followed the American starting ages for his schooling. We didn't send our child to ESF Primary mainly because of the class size issue and feedback we had from pre-school teachers that he was more of a 'small-group learner' who would do better in a smaller class size. However, we were more than pleased with the ESF facilities in our area and the teachers we met. I think I heard ESF described once as comparable to 'a good state school in the UK' but others with kids there might speak better as to that.
If you want a UK system then the Kellett school is probably something to consider. I believe date of application helps with the priority for acceptance they use so you'll probably want to get an application in there as soon as possible. Basically, schools have different 'priority' systems when moving through the applicants. Children of staff are often first, then parents who hold debentures, then siblings, then maybe date of applications, or passport, or parents are alumni, or whatever--it varies by each school but there are some that have date of first application in the priority so it helps to get it in to some schools sooner rather than later.
However, you really need to think about where you are going to work and where you are going to send the kid to school. A 45-minute bus ride for a kindergarten student is a bit rough to say the least, and some of the international schools are on the island itself (not sure where you are going to live).
As the articles linked above will attest, it can be very difficult, but if you have a few years to get ready and into the system, along with focusing on several different schools, you should find some options when the time comes (fwiw, we applied to 8 schools and got into 4, including our two of our top three).