From my research, it seems that US medical insurance cannot be used outside the USA without prior approval (usually only given for extreme emergencies, if then). This is definetly true of Bluecross. Calling your provider is the only way to confirm this, and since this is likely to be a very infrequently asked question, make sure you are getting an informed and correct answer.
Nor would your england-based insurance likely pay for care in the USA. You sort of have to pick. You can get generic Travel-health insurance to fill the gaps. The Lonely Planet Thorn Tree web page has a health branch where the pro's and cons of these insurance packages are regularlly debated. It's more geared towards breaking your leg while climbing mount everest then getting your teeth cleaned, on average. Only a very few travel policies offer coverage in the USA, due to the high cost of treatment there.
Of the main insurers over here, I chose BUPA. By USA standards, health insurance over here is a bargain, particularly if you choose a high deductable policy.
