Not the answer to your specific question but offered if it might assist.
Elderly Uncle died leaving estate to elderly niece. Niece not interested in funds; would rather to her children / adult grandchildren but with safeguards in place.
Deed of Variation provided a few immediate gifts but bulk to a dis trust; the trust then invested in segmented life assurance bonds thus no tax returns.
Annually, sufficient bond segments were appointed/assigned to children / grandchildren to fund an annual net contribution to pension. Niece (who nobody dared cross) made plain monies were for pension, and thus there was an annual ‘party’ wherein the deeds of assignment were signed, encashment forms signed by the appointees, and cheques collected from appointees in respect of the pension contributions.
Worked very well; the funds in the bonds funded about 10 years of contributions, and the original £100k ish turned into £250k ish of funds in pensions with earliest access aged 55 years.