Claim back additional costs
If you arrive at your destination only to find that your car hire company has overbooked and you no longer have a car, then there are a few things you can do.
Your first course of action is to see if the car hire company can supply an alternative car that you're happy to accept instead.
Of course, you may have to wait until a replacement can be offered to you.
Breach of contract
If an alternative car can't be provided and you're left stranded, or if you're not happy to wait for a replacement, then you can hold the company in breach of contract.
As such, you're entitled to claim back what you've already paid to the hire company. You're also entitled to hire an identical car elsewhere and claim back any additional costs.
Booking on a credit card
If a credit card was originally used to make the booking, you could also hold the credit card company jointly liable if anything goes wrong, under section 75 of the Consumer Credit Act 1974.
Section 75 covers items costing over £100 and up to £30,000, if at least a proportion has been paid on your credit card.
Or, if a debit or credit card was used and the sum involved was under £100, then you could use the chargeback system, where your provider can reverse the payment by trying to claw back the money you paid.
This isn't enshrined in law but many providers operate this system.