Car hire is complicated because most people don't do it very often and so they are to some extent at the mercy of the seller.
Basically, everyone says that they provide "full" insurance (although it is, just, possible to rent a car with just third-party insurance, and then you are on the hook for the whole value of the car if you cause an accident). This sounds great, but the problem is that "full" insurance is like your "fully comprehensive" insurance at home — it has an excess. And most of the shenanigans that you hear about is about the excess, which can be between £500 and £2000 but is most often around £1000.
There are several ways you can deal with the excess.
- Buy insurance at the desk when you pick up the car, to reduce the excess to zero.
- Do the same but when you make the reservation (often cheaper than 1).
- Leave a deposit for the amount of the excess, usually on a credit card. A debit card will usually not cut it. Check for the word CREDIT on the card. Every year, hundreds of thousands of tourists turn up with just a debit card and are told that's not acceptable for the deposit, and so they have to buy the car hire company's zero-excess insurance at the desk, and that's always going to be expensive.
If you leave a deposit, then more shenanigans begin with that. You can:
A) Perhaps find that permanent car hire deposit protection is bundled with the credit card
B) Get one-off deposit protection insurance
C) Get multi-trip (typically annual) deposit protection insurance
D) Take the risk.
In case of damage the car hire company will charge your credit card up to the amount of the deposit, and you will have to claim it back (A through C) or suck it up (D).
To claim the damage back, you will need to provide some specific documentation. The car hire company may or may not get a move on to provide you with all of that, in English, while you look nervously at your watch because your plane is going in 90 minutes. You may or may not fully understand exactly what bits of paper your deposit insurer needs.
Note that if you do have deposit protection cover (A through C), the car hire company does not care. They will still need the deposit on your credit card. This trips a lot of people up who rent through a broker site like Rentalcars.com. The broker sells you a cheap deal with Europcar or whoever, but both Rentalcars and Europcar make their real money by selling you insurance. So an hour after you make the booking, Rentalcars will be sending you scary e-mails about protecting your deposit, and so you give in and buy their insurance, but you don't realise that Europcar couldn't give a monkey's.
You also need to watch out for scams. Some companies, or just some unscrupulous seasonal staff who are after a bit of commission, will "discover" scratches on the car that were not marked on the damage sheet when you took delivery. Maybe it was dark, maybe you didn't look around. It's worth taking 20 or 30 photos of the car from every angle, but sometimes they "find" scratches under the bumper where you would never think to bend down. A few companies are notorious for this. There's a company called Green Motion who are explicitly excluded by many deposit insurance providers because it seems that their entire business model is charging ten consecutive renters £350 for the same scratches.