I am considering trading in my 2016 4 door for a 2025 or 2026 4 door S ....but really would feel more comfortable if i can put a donut spare in the boot,
preferably under the floor of the boot, if there is an under on the S ....has anyone tried this ?

 

I don't have a 4 door, but a donut spare will not fit in the compartment in the rear of my 2025 F66.
There is space for stuff, but not anything like a donut (except the kind you eat).
With the back seat down, I could place a donut spare above the compartment.
For a long trip, this might make sense.
Like you, I would prefer to have a donut spare, just to be on the safe side....

 

Donut spare won't fit in our 4 door. We have a Mercedes collapsing spare and even it won't fit when deflated.