Are you asking why things cost more when the pound is weaker compared to the dollar? If so, here's a simplified example...
Say last year Tesla makes the car for $10 and it retails for $20 in America, they make $10 profit per car.
The example exchange rate then was even, £1 = $1. So the $10 cost to make the car equals £10. They can sell it in the UK for £20, and make £10 profit on each one, which equals $10 profit, same as in America.
Then, 6 months later, the £ drops by half. £2 is now only worth $1, so that $10 car now costs the equivalent of £20 to make. Tesla also still want to make $10 profit per car, which also now equals £20.
So it now sells for £40 in the UK, instead of £20, to make the same $10 profit.