Is this the right way to plan my next car purchase? Mortgage Borrowing?

IAMIAM

I usually buy a car for 15-20k and have it for a good 10 years.

Mortgage 160k
Additional Borrowing 20k
Total Borrowing 180k at 4.74% Tracker HSBC from 1st July (Hoping its 4.49/4.24 by Summer)

Purchase car on BMW PCP, highest deposit, over 4 years, mileage say 15k. Then once agreement up and running, week later, overpay again by the maximum and have a monthly finance agreement of say £50 a month with a view of paying a balloon payment at the end of the 4 years or indeed cancel and return the car.

Whilst that is happening, pay my Additional borrowing on mortgage at say £400 a month so its cleared in 4-5 years....

grumpy_codger

Define "right way".
And I'm no expert in car finance, but I don't understand the plan. If the car is £20K and you borrow £20K against your house, why do you need car finance? Is it better than just buying a car and then selling it when/if you want?