no probs... hmm. It all depends what date they put as your finishing date on your P45. THAT is the date that your P11D will use to confirm your time in their employment and, therefore, in receipt of a car as a taxable benefit... I guess that once you have your P45 you will be able to make a decision.
Also, note, they will fill in your P11D for the time you were with them and should send you a copy in the post (when the end of year tax returns are filed) but the dates should tally with the date on your P45, if not then you will have to contact your tax office at that time and dispute it.
You are entitled to holiday pay, it is mere coincidence that you are leaving in the midst of bank holidays.
If you had booked those 3 days as holiday and normally get holiday pay for them, and in your letter of resignation your final day is stated as Dec 29th, and it is on that basis that they accepted your resignation, then that is that. You are still in their employ until the 29th and your P45 should reflect this. You should receive your P45 with your final pay cheque so will be able to check then.
IF the date on your P45 is the 22nd and they have only paid you to then, they will owe you an additional 3 days pay.
If the date is the 22nd and they have paid you the 3 days then you will have to return the car on the 22nd as that will be the date submitted to the Inland Revenue, although they are then accepting only 3 weeks notice (effectively) and you could argue that they are in breach of your contract and denying you a weeks wages.
Hope this helps and that you get it all sorted out ok. not good to leave under a cloud, but you have to stand up for your rights and if you are going to be paying for the car to the 29th then you damn well keep it until then!