The tax and the MoT aren't linked. So you'll need to reclassify your vehicle as historic for VED purposes. Then, next time you tax it online, you can check the MoT self-declaration box. Good idea on getting it checked every year, especially as your insurance won't be valid if the car isn't in a roadworthy state. Just remember, though, that if it does go for an official test and it fails the same rules apply (i.e you'll need to get the fault rectified and the car retested for it to be legal).