tl;dr
Your rental car will come with adequate insurance to ensure its road legal and that the rental company won’t lose out. See this See this handyhandy rental guide for additional insurances you may be interested in, particularly point 5.
Every vehicle has to have an insurance policy in the UK if it’s to be driven on the roads. That’s a legal requirement. As such all rental cars come with at least some level of insurance cover (rental companies don’t want their cars impounded because you didn’t take out insurance). Any other insurance you choose to have is in addition to this (though not legally required).
There are three main levels of cover in the U.K.
Third Party
- The minimum cover required by law in the UK. It covers you against costs that arise as a result of injuries you cause to other people and damage to their vehicles.
- I believe this is equivalent of what you call liability insurance. It covers your liability against third parties.
Third Part Fire & Theft
- As above with added protection against your vehicle being stolen, or destroyed in a fire.
Comprehensive
Third Party Fire & Theft plus cover for damage or theft of your vehicle's contents.
I believe this is similar to your collision insurance.
Your rental car should come with comprehensive insurance, I’ve never heard of any company providing less.
There are many other additional insurance add-ons you can purchase but these aren’t legal requirements. One they car rental company will probably try and sell you is excess waiver insurance. If you take this out and subsequently need to claim on the insurance policy, there will be no excess to pay. The catch is the insurance they sell you can cost a significant amount. I would recommend taking out your own excess insurance prior to picking up the car. It’s easy enough to take out your own policy online and significantly cheaper.