Just offer a kind polite version of the truth.
"I'm having to adjust our diet for family reasons, and I'm afraid Christmas Pud doesn't suit some. Next year we're going for a lighter dessert everyone can share. So this is to thank you for all the past years Puddings, (and let you know in good time, so next year you can put some other lucky person on the pudding list).
Deliver it in a card with a bunch of flowers. DDo it in the next few weeks.
If she probes about diet, you just say " I can say no more, apparently it's a very sensitive issue" (roll eyes, smile, change subject).
Next year if she offers/ delivers a pud you've laid all the ground work and can just say firmly " No thanks, please give it to someone else. No, I really mean it. ".