My children attended Tanglin Trust School (
http://www.tts.edu.sg) and United World College of South East Asia. (
http://www.uwcsea.edu.sg) Both schools are excellent with superb facilities and comparable with private schools in the UK. Although UWC is an International School its origins were that it was the secondary school for the children of the British Army; hence the majority of teachers are British but also from mainly Commonwealth countries. It has the GCSEs/IGCSEs, as does Tanglin.
My kids were at the local village C of E primary school in England before we moved to Singapore but they fitted in very well - helped by the schools having a 'buddy' system to help new arrivals. Both schools have lots of extra-curricular activities and the school trips are amazing - eg. a week in Borneo in Year 5 including meeting an Iban (ex-head hunting) tribe and sleeping in their longhouse!
Note that both schools are 'Trust' schools which are non-profit making and all income is ploughed into the schools and not into the back pockets of the Directors. Just go to the other 'profit-making' schools and you will see that the facilities are much poorer in quality. The only other Trust school in Singapore is the Singapore American School.
The only problem I can forsee is that Tanglin Trust and UWC are so popular that they often have long waiting lists in certain age groups and it is not uncommon for some children to have private tutoring at home for a term until they can get in. However, the big expat turn-around is in the Summer at the end of the school year so you may be lucky.
If you can't get your kids in there I have heard positive things about the smaller Chatsworth International School and I do know some Brits who couldn't get their kids into TTS/UWC and their son was happy at the Australian school (but the academic year/holidays were a pain for them).
Hope this helps!
