The trouble is that the charges that the SIPP operator levies are likely to swamp the funds available to invest. One of the cheapest SIPP providers (for a simple SIPP) is Liberty. Take a look at their website to see their charges, remembering that these are entirely separate from the investment management fees. Another low-cost provider is Alliance Trust Savings, and while they are themselves investment managers (the investment trust Alliance Trust, and probably others) you are not restricted to investing in their funds.
Nothing in this post is to be construed as investment advice. The I (and my wife) have SIPPs with Liberty, and also manage a SIPP on behalf of our grandson through Alliance Trust Savings. Until retirement I was an adviser to private investors, but relinquished my registration on retirement.