UK Pension

What pension? If it is the State pension, then there is lots of information here:
http://www.hmrc.gov.uk/international/personal.htm
http://www.hmrc.gov.uk/ni/volcontr/abroad.htm
If you eventually plan to retire in the U.K., then making voluntary NI contributions can be a good idea. However, the contributions you make here in ONSS can also be counted towards a U.K. state pension, and you don't want to pay out twice for the same thing. Contact the helpdesk for non-uk residents here:
http://search2.hmrc.gov.uk/kb5/hmrc/contactus/view.page?record=OLul3tPejl4
If you are talking about a non-state pension (like a company pension), then the only people who can really tell you if you qualify, or if they can take your money, will be the pension fund administrators.