Hi David,

You should talk through the intricicies of this transaction with your accountant, as it sounds to us as if you are, effectively, giving the company your existing car, which would probably be treated as a loan from you to the company. You also need to check with the dealership to ensure they will accept such an arrangement, as it may be that they require the trade-in to be in the same name as the new buyer. 

At first glance, I suspect it'll be easier for you to officially sell your car to the company first (make sure the price is realistic or you'll be liable for BIK), but as I said, this really is something that you should discuss with your accountants before making any decisions.