Using multiple credit cards for a single payment in the UK (section 75 protection)

How can someone buy a product which is over each of its credit card limits, but less than his total credit?

Suppose that I want to buy a car, which is £4500, and suppose that I have three credit cards from three banks with credit limits of £2000, £2000, and £1000 pounds.

Also, suppose that I do have the money to buy the car. Therefore, I don’t want to pay interest, and that I will pay the entire bills in the first month for all my bank.

The whole purpose of buying the car in a single transaction is to be protected by Section 75 of the Consumer Credit Act, meaning that if something goes wrong, I have more protection.

The problem in most shops/websites, you need to pay it in a single transaction. You can’t pay a portion of the price with a credit card and the rest with another. What kind of fees am I expected to pay for "merging" these cards for a single transaction?

I am mostly thinking of keeping the three cards and not merging them indefinitely. Instead, I only want to merge them for ONE transaction, and then still having three cards.

Any suggestions?