I sold car A for £42k paid to me as £22k cash and car B (£20k) in part exchange which I then sold for £22.5k. How do I record the sale of vehicle A and B & purchase of B?

On the assumption you're not a car dealer, then the £42k would be posted to an expense account - profit/loss on sale of motor vehicle, you'd then have to take the net book value of that car out of your fixed assets and debit that expense. The profit of £2.5k would likewise end up in this account.

If you are a car dealer the these numbers would end up in sales and purchases.