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.