I’m in the Middle East, and there are a number of families I know who have done this.
The main reason is that the main breadwinner can’t really give up their job - they are too close to retirement to be considered by another company. Leaving a job voluntarily gets increasingly risky as you approach 50. This is coupled with the fact that the jobs market in Britain isn’t great at the moment.
If they were to go back, it would normally mean a big salary chop, which would mean state schools for the kids. The UK’s education system (rightly or wrongly) doesn’t look great from a distance, so people worry about putting their kids into it.
There is also an element of people having ‘done good’ out here and have maybe come from quite tough backgrounds to suddenly having more money. It’s an exciting option for them to be able to give their kids what they perceive to be a social boost by sending them to boarding school.
Finally, one of the BIG factors here in the decision is getting kids used to normal life in England and allowing them to develop a bit of street sense in a gentle way, before they go to Uni and learn the hard way. I don’t know about Asia, but kids here grow up very naive - they are less exposed to drugs, alcohol, crime and general teenage naughtiness. So sending them back for sixth form is a way to gently overcome this.
Another factor is the quality of education. It’s a vicious circle really; because so many families do this then sixth form provision can end up being quite small in many schools, or the numbers are boosted by non-native English speakers. Thus, the quality and results may not be the same as those obtained in the U.K.
Finally, there is the issue of residency to consider 3 years before uni. If you’re not U.K. resident for 3 consecutive years immediately before going to a U.K. university, you might have to pay overseas fees, which are considerably higher.