Firstly, not that it makes a difference to the answer, he's not a Ltd company subject to corporation tax, but a
self employed GP
Secondly,
I would say it was prudent to show this income while at the same time showing the correct value of the asset in the BS.
That's all very well, but you're subjecting him to possibly as much as £1,220 of tax by treating it as a grant. It cannot possibly be correct that the action of buying a car which has allegedly been subsidised by the Government for the purposes of boosting the economy should give rise to a tax bill of £1,220, i.e. more than the subsidy from the Government - notwithstanding the point that it would only be a timing difference. It's neither income nor a grant. It's a discount to cost.
I dont see the difference between a parson paying a £500 deposit personally or using a personal asset as a deposit. Either way its £500 from the person and is certainly not liable for tax relief.
I suggest you have completely misunderstood capital expenditure and capital allowances. Cash laid out on capital expenditure certainly IS eligible (I don't think you meant "liable") for tax relief. If it isn't, under what circumstances do you think that you ARE elegible for tax relief?????!
Finally, perhaps you would explain how you think he would get AIAs. In my copy of the legislation there are no AIAs for cars, see s38B General Exclusion 2 CAA 2001. Under the law as it applies to me there is a real tax cost to him of your suggested treatment of £2,000 as income (unless it is a green car and 100% FYAs apply).
So, in conclusion, I remain absolutely convinced that it is correct to put the car in at a capital allowances value of £5,600 PLUS the value of the car traded in. And this value is not nil, as the car has to have a valid MOT at the point (or very shortly before) it is traded in.