There are many SIPP administrators obviously, but let's say you wanted to buy gold through GoldMoney. They work with BerkeleyBurke.
Here is more information: http://goldmoney.com/gold-sipp.html
For a SIPP they want to see 50,000 (the reason mostly being the high fees, is my guess). But they also have something called GoldPension, which is like a SIPP, only you can buy bullion only, but there is almost no lower limit: http://goldmoney.com/bbsipp/bb-brochure.pdf
So, even if you only go for the "GoldPension" scheme, you can put all your pension money in gold bullion via GoldMoney.