I haven't read the whole thread so apologies if anyone else has suggested this - but in your place, @WrackspurtsAndNargles62442, I'd tell her you'll sort out the cake, and get your cake maker to make her a special little fruit cake, just for her and those other guests who'd like it.
That way, you can make sure it is decorated to your taste, and matches your proper wedding cake, and your MIL will be happy too. And if it is made by your baker, you can ask her to feed it and put plenty of fruit in, so it will be much nicer than a bought cake from M&S.
This way, I don't think it would take anything away from your wedding or your cake, but would add on a little extra that would keep the peace with your MIL.
Obviously, if she is an utter nightmare of a MIL, don't do this - previous posters are right that it would just encourage her - but if she is, in general a nice person, and you get on well with her, it might be worth giving a bit in this specific instance.
Also - to all the posters who have said fruit cake is dry and nasty - you've never tasted my Christmas cake! Admittedly it is pretty alcoholic, because I soak the fruit in brandy for a couple of weeks before making it, and then feed the cake with more brandy several times before marzipanning and icing it - and I put in loads of fruit, in an all-butter cake mix - and it is moist and delicious, I promise. Fruit cake doesn't have to be dry or dull.