Inherited US Annuities

fI am having trouble determining if the accounts I inherited from my American mother are taxable in the UK and if so, how the tax would be calculated and reported. The accounts were non-qualified annuities. Tax was withheld on a partial amount in the US. They were cashed in and I received a lump sum payment five years after my mother died, as I was advised was necessary and I have received the 1099r forms for them. I'm trying to ensure this is done correctly but it's not easy so I hope someone here can help answer or direct me to the answer, such as an article in the UK/USA Double Taxation Agreement or something similar. What I found in the agreement seems to apply to distributions, not a lump sum.

Hi 

Foreign annuities are declared in a self assessment tax return in the foreign section.  On paper supplementary page SA106, boxes 43 to 45.
Foreign (2024)
Foreign notes (2023-24)

Thanks