Why do UK citizens pay more for VOA

Rick, the fee is charged based on the Mutual Visa Policy. UK also charges more for Chinese citizens when they apply for the entry permits.

In that case the Mutual Visa Policy is unfair and needs updating. The fact that you mention the rate of UK enttry permit charges gives the impression that your high fees for UK citizens are more to do with "tit for tat" that what is just. Incidentally UK rates for Chinese visitors especially students have been considerably improved just recently and China should follow suit without further delay.