score:6
I can confirm that is possible and easy to add a foreign visitor with some insurance companies at least. I have temporarily added friends and family to my insurance a number of times. One point for the owner to consider is that his no claims discount could be affected if you need to make a claim. So, your friend would need to have a high level of trust in you. Even if the car is worth little, you could cost him a lot.
One thing that you should not do is simply get in the friend's car and drive. It is very rare for UK insurance policies to cover any driver. Most only allow the policy holder and specific names only.
Policies here often offer cover for the policy holder to drive other cars. However, even when the policy is comprehensive for your car, this cover is usually only third party. Your home policy might have something similar but check carefully that it would be valid here. If you have an EU policy then it might be okay but still check carefully. If you have a non-EU policy then I doubt it but you could ask. Either way, bring some good evidence of cover if you do this.
Upvote:4
By far the easiest way to do this is to get your friend to add you to his insurance. This is usually a very low cost option - some insurers will do it for free in some cases. Once done you are totally covered and legal to drive.
If this is not an option for some reason, you might try getting this added to your insurance. Some companies will allow you to add coverage for overseas rental cars, and they might stretch it to a borrowed car.
Failing that, your own short term coverage.