I don't know which school your children are in but there is a small number of people people from Dubai whose sons have successfully applied to Eton and Winchester and plenty of other, less selective schools without using a prep school. Takes quite a bit more effort from you though to ensure you are familiar with what they want - not at pre-test level which is largely an iq type test and interview, and should not pose a problem if your child is of sufficient academic calibre, but later, when doing Common Entrance as the syllabus differs somewhat from National Curriculum. Some tutoring would be needed.
The gap between NC and core CE subjects (Maths, English) is not much if you are at a top Dubai school as standards are high, but Latin provision can be a problem, even though all schools, even Eton, allow for a lower tier exam to be taken in this subject in certain circumstances - and French teaching is different and, I am told, of a lower standard than at prep schools. The other problem is that Dubai schools are not gearing up for exams at the end of year 8 as prep schools are so exam practice is not available in school (except at Repton, but I don't know much about Repton's standards).
It is not uncommon for anxious parents to send 9 year olds to well known Uk preps in order to get into top schools and to find that their "third set for maths" son is placed immediately in top set at his new prep. If you don't want too much work and want to keep your child with you until 13, I believe that as well as Canford, Charterhouse is very understanding of overseas children who have not attended traditional prep schools. Many other schools will be too if you ask them.
The decision about leaving for prep school depends entirely upon the school your child is currently attending in Dubai, however, as only a tiny number cut the mustard in this respect.
If Eton or Winchester are your aim you need to be well organised, well informed, and self disciplined if you decide to keep you child with you after the age of 10. It is possible though.