When I was a teen and didn't own a car, I was added to the family car insurance plan. So that makes sense to me to have insurance without a car (if you have access to a car).
But no, the UK is different. Some insurance companies may take your US driving history into account, so it's worth getting a letter from them. Most will not though. Be prepared to be treated as a new driver for the first few years.
I would just use an insurance comparison website. moneysupermarket is a good place to start. But if you don't have a car, you can't buy insurance. You can be added to someone else's vehicle though. In the UK, 99.9% of the time, you can only drive a vehicle which you are a "named driver" for and on the insurance. It's not like the US where if you have permission to drive the vehicle, then you are covered.