Cheques are notoriously bad for guaranteed payment - businesses would only accept a cheque if your bank also issued you with a cheque guarantee card (usually just a different design on your debit card), which means the bank would guarantee to cash the cheque up to a certain amount, taking the matter up with the writer of the cheque if it bounced. If you tried to cash a cheque without a guarantee (or a cheque over the guarantee amount) and it bounces, you are SOL and have to take it up with the writer yourself.
And cheque guarantees usually only went up to a few hundred quid.
Car dealers would typically want a bankers draft, which is a bit of paper issued by your bank for a specific amount and is treated as cash - the value is held on the paper, its not an instruction to transfer money, its an actual promissory note just like paper note cash is. Lose the bankers draft, and the money is gone, you can't get it back.
Bankers drafts cost you money to buy, and you have to go to a bank to have one issued.