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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

BeanSmart.com is a site by and for consumers of financial services and advice. We are not affiliated with any of the banks, financial services or software manufacturers discussed here. All logos and trade names are the property of their respective owners.

Tax and financial advice you come across on this site is freely given by your peers and professionals on their own time and out of the kindness of their hearts. We can guarantee neither accuracy of such advice nor its applicability for your situation. Simply put, you are fully responsible for the results of using information from this site in real life situations.