Depending on your salary, the tax deduction will vary.
For detailed information, please refer to the National Tax Service site in English: http://www.nts.go.kr/eng/
Pension status for citizens of the U.K.
Unlike the US and Canada, Korea and the UK have only agreed to a 'contribution only' convention, at this time. What this means to UK citizens working in Korea, is that your Korean pension contributions can be added to your accruing pension credits in the UK, of which you can begin to receive when you retire in that country.