If you're the policyholder or NCD holder with LV= comprehensive car insurance, you'll usually have third party cover to drive other cars as well. Have a look at your policy documents to double check you're covered and make sure you've got the owner's permission to use their car first. Please be aware that you're only covered to drive other cars in the UK.
If you're on holiday and want to know if you're covered to drive a hire car, depending on the country you are travelling to, you may be legally obliged to have three types of basic insurance. These are usually included in your car hire costs and contract.