If you paid for the car on hire purchase (HP) then you are perfectly entitled (no small print clauses) to hand it back when the total repaid (including the deposit) is half or more the original purchase price (less any accessories). They are entitled to ake deductions for excessive wear and tear/damage, but not for excessive mileage.
If you bought it on a loan, then there is no recourse whatsoever as far as I am aware.
I did the same a few years ago – just signed a form and gave them the keys. Went out and bought a brand new, better car for the same monthly repayments over just one year more term than I had outstanding – simply by getting a car on a much, MUCH better finance rate (I was rammed unmercifully on the first tbh, but I was young and stupid).