I have done the LHR-Cali (LAS) route every year now for 10 years.

It's certainly possible but don't hope for a direct route, for me it has almost always ended up with Los Angeles as it seems to be the easiest redemption route. Well direct flight to LAS is also a pretty new thing

San Francisco and Las Vegas almost impossible.

Look for Los Angeles, Denver, Houston, Seattle, even Chicago and New York. You can then add a cheap AA KUP ticket and still travel in style, and collect massive TP ^