According to the ESF 2015-16 annual report, out of the 12,899 ESF students, around 20% held the British passport. It works out to be 2580 students.
http://www.esf.edu.hk/wp-content/uploads/2017/01/ESF-Annual-Report-2015-16.pdf
According to Wikipedia, the whole school student population in Kellett is 1200, assuming every single student is Brit (can't be true in reality), there are more Brits in ESF schools all together when compared to Kellett School.
The reason why Kellett School is one of the two British schools in HK is that it is a member of the Federation of_British_International_Schools_in_Asia FOBISIA.
Although King George V School was founded as Kowloon British School (c. 1900), it is not a member of FOBISIA, but rather with English Schools Foundation.