SIPP

A Self-Invested Personal Pension (SIPP) is the name given to the type of UK-government-approved personal pension scheme, which allows individuals to make their own investment decisions from the full range of investments approved by HM Revenue & Customs (HMRC). - From Wikipedia

When an investment is advertised as being SIPP "approved" this doesn't mean in any way it is a good or safe investment for your pension. It only means that your are allowed to include this into your personal pension scheme.