Cash in SIPPs and ISAs

Happy New Year all.
We can get reasonable interest on fixed rate cash bonds, eg 3.1% for 5 years.
It annoys me, and I think it outrageous that banks and BSs offer lower
rates for cash in ISAs. Any "good" reason, other than they can get away with it?
I would like to hold cash in some of my SIPPs in fixed rate bonds, eg the 3.1% for 5 years - but this seems impossible. I assume the reason why these are not offered is there is little in it for Hargreaves Landown, Cavendish, III, etc, as these rates will be very competitive. OK, the ~0.5% SIPP fee would come off, but I could live with that.
Is there a way of holding cash bonds in a SIPP? Does any SIPP provider offer this? The only way I can think of is to buy (relatively short maturity) Treasuries, but those rates are worse than retail cash bonds.
I'm just trying to make my SIPP portfolio a bit more defensive / conservative.
Thanks in advance, Fred
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