Heya...
We just moved to the UK (well actually are in the process, visiting family in Canada now but will be back on 2 Jan) from the states. I did some online estimators and ended up trying (on a lark) the RAC insurance. I figured, RAC, good brand, etc... couldn't be too bad, right? I went through the online application, and at the very end, under the Terms and Conditions, I found that you have to be a UK licensed driver.
So I phoned up their number, and the phone sales person said no, that it was no problem, they would honour your US license until you got a UK one. They also gave us credit for our no-claims in the US. Once all was said and done, it was about £450 for a year's cover for me and my partner, and that was comprehensive cover on a 2000 model Saab that is parked on the street in London. And they let you pay over time.
Which I thought was quite good--that's a bit less than we pay on our '95 Volvo in the Seattle area.
-B