CLeung, if you are not sure how long you will be staying and have a child who prefers co-ed I would plump for a 13+ prep, ie Newton Prep, Thomas', or Fulham Prep. Or Hill House mentioned above. I would also try Garden House, Weatherby, Eaton Square etc. There is quite a lot of population movement in Central London. If you say you are looking for a place till 13 they are more likely to be interested, should an occasional place come up.
There appears to be a view amongst a section of the London ex-pat community that a place at Colet Court/WUS is the be-all and end-all. True Westminster and SPS are fantastic schools, but for the right boy. What can be a fantastic and challenging environment for one, becomes a stress inducing hot-house for another. The education arms race is such that a lot of 6 year old boys are being prepped very heavily, and this can then continue for the 11+ pre-test, and then onto 13+ Common Entrance. Enough to turn a bright but not outstanding boy off education for life. Really not worth it unless you want to stay in the English system for the duration.
The schools I have mentioned are large schools, so lots of scope to set and stream. A good cohort will stay through to 13, and will expect to receive a good consistent education, rather than the stop/start of being prepped for 7+ and 8+. All of these schools regularly send pupils to the big name secondaries, but also to a range of other good schools, giving you time to find out what sort of child you have first. Some children perform much better if they are towards the top of their cohort, and will thrive better at say, Latymer Upper, Alleyns, Highgate or Hampton, than if they are in the bottom third at SPS or Westminster.
Your son is already having to change country and education system. Do you then really want him to be working flat out for external exams so he can change schools again, and then find that you are moving country. If you are still around at 11, he can do the pre-tests, and if you then move country to somewhere where the international school is not well regarded, you have the option of keeping him in the British system via boarding.
Before deciding on a school it is worth looking at TfL's journey planner. Its time not distance that matter. You dont want to have to drive, so a school on a direct tube line or bus route should be preferred.