There are many good SIPP providers out there, like BestInvest, Interactive Investors and Vanguard investor. FreeTrade would be my go-to platform but it does not allow Limited company (employer) contributions just yet. Interactive Investors offer a nice balance between fees and user interface. The Vanguard SIPP is very good. Their 0.15% platform fee is reasonable if you can live with the Vanguard-only fund selection.