I don't know what a self invested pension is. I hold Bitcoin because I want self custody of my money.
In the UK most workplaces are required to pay into a pension plan for employees so everyone has work place pensions that you cant touch until retirement. More savvy people transfer those funds into SIPPs so they can at least control what that money is invested in like MSTR etc.
It’s a UK pension pot that is self invested. MSTR is the only way you can use a tax efficient vehicle to invest in Bitcoin. ETF exposure is banned. Direct exposure to Bitcoin is also made very difficult - it’s almost as if they don’t want you to own it - it’s for your own protection. 😉