@kezo, illegal, no. If an item that isn’t subject to VAT is added to a product that will be, then VAT will be added to that new product in its entirety. This is what happens when you take insurance and roll it up as a ‘just add fuel’ product. However, if you have a car with suitable adaptations, you should still be able to claim an exclusion from VAT.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark