When I lived in London, I had a US Touring Auto Policy from USAA. If I recall, it is very inexpensive (<$50 per year). If you rent a car when you are back in the US, you get basic liability coverage of ~$100k. However, its most important function is allowing you to maintain continuous coverage and not have an insurance gap. If you return to the US in a few years, your driving record will look great (assuming no incidents) and you will be eligible for lower rates.