If the hire car you've booked is not available, lodge a complaint with desk staff immediately. Then follow it up with a letter to the company as soon as possible.
I booked to hire a [insert car type] with you [dates, location, and price].
When I arrived to pick up the car I was told that the car that I had hired was not available and all of the cars were fully booked.
I complained immediately to your representative but the matter was not resolved to my satisfaction.
It was an express term of the contract between us that I would be provided with a [insert car type] on [dates, location, and price].
Your failure to provide any car constitutes a breach of that contract. This caused considerable disappointment and inconvenience and I had to hire an alternative car.
I am entitled to compensation for the consequences of your breach of contract.
As I experienced [problem], I look to you to pay me [£xx] in compensation, calculated on the basis of [details of the cost of hiring an alternative car].
I look forward to receiving your proposals for reparation within the next 14 days.
If I do not hear from you with a satisfactory response to this letter within 14 days then I will proceed to start legal action against you in accordance with the civil procedure pre action protocol.