If you are financing at £1k a month a large element will be capital repayment part of which is covering the cost of depreciation. If you borrow 50k and interest rates are 5% you are paying about 200 a month interest so 800 is capital reduction. At the end of 18 months you have repaid over £14000 way more than the depreciation in my case...Simple isn’t it!!
In your personal scenario, you buy your car outright with cash, own it for 18 months and you trade it in for a new car you're buying with cash, the cost of depreciation would in fact be your quoted £6,600.
In a more typical scenario of the same initial car purchased for £60K, financed with a £10k deposit and £1000/ month which you trade it in for a new car 18 months later, lets say you're buying cash for £60k to negate the speculation of hidden incentives. Regardless of interest and capital percentages, the cost of depreciation is £10K + £18k +£6,600. Unless you were financing the new car for a significantly less monthly commitment.
That's how I look at it, but that's why I'm a salesmans nightmare.
Anyway, enjoy the new beast!