In order to qualify for UC you have to have savings lower than £16k. SIPPs and other pensions are not counted towards those savings. However, you and your husband have to make a joint claim for UC, and the eligibility rules are applied across your savings and income as a couple.
In theory, even though he isn't working, your husband could move funds from his ISA to his SIPP, depending on the rules of his SIPP for making payments. However, bear in mind that once the funds are in a SIPP he cannot take them out again until he is 55.
Because of the Joint savings rules, you would have to move your ISA into your SIPP as well. Due to the access date for SIPPs changing in 2028 you would not be able to access the funds in your SIPP until you reach 57 (under present rules - those might change in the future, but are more likely to go up than down...)
However, I think that your work income is too high for you to qualify for UC.
This link points to how it all works and a benefits calculator.
Even if you did qualify you also need to consider that when you start taking your money out of your SIPPs that counts as income for tax purposes, which withdrawals from an ISA do not.
All in all, I think it is possible to do what you are suggesting only by cutting your income and savings and you would be sacrificing flexibility and making yourselves a hostage to changing Government regulations - personally I would avoid that in the current economic and political circumstances.